Michel Caffarel

Thomas Applencourt

Emmanuel Giner

Anthony Scemama

Using CIPSI nodes in diffusion Monte Carlo

Several aspects of the recently proposed DMC-CIPSI approach consisting in using selected Configuration Interaction (SCI) approaches such as CIPSI (Configuration Interaction using a Perturbative Selection done Iteratively) to build accurate nodes for diffusion Monte Carlo (DMC) calculations are presented and discussed. The main ideas are illustrated with a number of calculations for diatomics molecules and for the benchmark G1 set.

I. INTRODUCTION

In recent years the present authors have reported a number of fixed-node DMC studies using trial wavefunctions whose determinantal part is built with the CIPSI approach. [1][START_REF] Giner | Méthodes d'interaction de configurations et Monte Carlo quantique : marier le meilleur des deux mondes (Configuration Interaction and QMC: The best of both worlds)[END_REF][START_REF] Scemama | [END_REF][4][5] The purpose of this paper is to review the present situation, to clarify some important aspects of DMC-CIPSI, and to present some new illustrative results.

In section II we briefly recall what Configuration Interaction (CI) methods are about and present the basic ideas of perturbatively selected CI approaches. We emphasize on the very high efficiency of SCI in approaching the exact Full CI limit using only a tiny fraction of the full Hilbert space of determinants. Selecting important determinants being a natural idea, it is no surprise that it has been introduced a long time ago and has been rediscovered many times under various forms since then. To the best of our knowledge selected CI appeared for the first time in 1969 in two independent works by Bender and Davidson [6] and Whitten and Hackmeyer. [7] In practice, the flavor of SCI we employ is the CIPSI approach introduced by Malrieu and collaborators in 1973. [8] CIPSI being our working algorithm for generating CI expansions, a brief description is given here. It is noted that the recent FCI-QMC method of Alavi et al. [9,10] is essentially a SCI approach, except that selection of determinants in FCI-QMC is done stochastically instead of deterministically.

In section III the performance of CIPSI is illustrated for the case of the water molecule at equilibrium geometry using the cc-pCVnZ family of basis sets, with n = 2 to 5 and for the whole set of 55 molecules and 9 atoms of the G1 standard set. [11,12] It is shown that in all cases the FCI limit is closely approached.

In section IV the use of CIPSI nodes in DMC is discussed. We first present our motivations and then comment on the key result observed, namely that in all applications realized so far the fixed-node error associated with the approximate nodes of the CIPSI expansion is found to systematically decrease both as a function of the number of selected determinants and as the size of the basis set. This remarkable property provides a convenient way of controlling the fixed-node error. Let us emphasize that in contrast with common practice in QMC the molecular orbitals are not stochastically re-optimized here. An illustrative application to the water molecule is presented. [5] Of course, the main price to pay is the need of using much larger CI expansions than usual. The main ideas of our recently proposed approach [13] to handle very large number of determinants in QMC are presented. In practice, converged DMC calculations using trial wavefunctions including up to a few millions of determinants are feasible. The computational increase with respect to single-determinant calculations is roughly proportional to ∼ √ N dets with a small prefactor.

In section V the implementation of effective core potentials (ECP) in DMC using CIPSI trial wavefunctions is presented. As already proposed some time ago, [14,15] CI expansions allow to calculate analytically the action of the nonlinear pseudo-potential operator on the trial wavefunction. In this way, the use of quadrature points to integrate the wavefunction over the sphere as usually done [16] is avoided and a gain in computational effort essentially proportional to the number of grid points is achieved. The effectiveness of the approach is illustrated in the case of the atomization energy of the C 2 molecule.

Finally, Sec. VI presents a detailed summary of the main features of the DMC-CIPSI approach and some lines of research presently under investigation are mentioned.

II. SELECTED CONFIGURATION INTERACTION A. Configuration Interaction methods

In Configuration Interaction the wavefunction is written as a sum of Slater determinants

|Ψ = i c i |D i (1)
where determinants are built over spin-orbitals. Let {φ k } be the set of N MO orthonormal molecular orbitals used, the size of the full Hilbert space is given by the number of ways of distributing the N ↑ electrons among the orbitals times the corresponding number for N ↓ electrons. The total size of the full CI space is then (no symmetries are considered)

N F CI = N MO N ↑ N MO N ↓
The CI eigenspectrum is obtained by diagonalizing the Hamiltonian matrix, H ij = D i |H|D j within the orthonormal basis of determinants. In practice, the exponential increase of the FCI space restricts the use of FCI to small systems including a small number of electrons and molecular orbitals (N F CI not greater than about 10 9 ). To go beyond, the FCI expansion has to be truncated. The most popular strategy consists in defining a subspace of determinants chosen a priori. Typically, the Hartree-Fock determinant (or a few determinants) is chosen as reference and all possible determinants built by promoting a given number of electrons from the HF occupied orbitals to the virtual ones are considered. In the CIS approach only single excitations are considered, in CISD all single and double excitations, etc. Now, numerical experience shows that among all possible determinants corresponding to a given number of excitations, only a tiny fraction plays a significant role in constructing the properties of the low-lying eigenstates. Furthermore, the weight of a determinant in the CI expansion is not directly related to its degree of excitation. For example, quadruplyexcited determinants may play a more important role than some doubly-or singly-excited determinants. However, in practice, limiting the maximum number of excitations to about six is usually sufficient to get chemical accuracy. To give some quantitative illustration of these statements, Figure 1 As a consequence of the preceding remarks, it is clear that it is desirable to find a way of selecting only the most important determinants of the FCI expansion without considering all those of negligible weight (the vast majority). This is the purpose of selected configuration interaction approaches.

B. Selected CI and CIPSI algorithm

To the best of our knowledge Bender and Davidson [6] and Whitten and Hackmeyer [7] were the first in 1969 to introduce and exploit the idea of selecting determinants in CI approaches. In their work Bender and Davison proposed to select space configuration using an energy contribution criterion. Denoting |φ 0 the restricted HF CSF-configuration, |φ l i all possible spin configurations issued from the space configuration, and

i = 1 k k l=1 | φ l i |H|φ 0 | 2 φ 0 |H|φ 0 -φ l i |H|φ l i (2)
the "average" perturbative energy contribution, the space configurations were ordered according to this contribution and those determinants contributing the most selected. The CI wavefunction was then constructed by using the selected configurations, |φ 0 , and all single excitations. A few months later, a similar idea using the very same perturbative criterion was introduced independently by Whitten and Hackmeyer. [7] In addition, they proposed to improve step-by-step the CI expansion by iterating the selection step to reach the most important determinants beyond double-excitations.

In 1973 Malrieu and collaborators [8] presented the CIPSI method (and later on an improved version of it [17]). In CIPSI the construction of the multirefence variational space is essentially identical to that of Whiten and Hackmeyer. However, in order to better describe the dynamical correlation effects poorly reproduced by the multireference space, a perturbational calculation of the remaining correlation contributions was proposed. In applications the perturbational part is usually important from both a qualitative and quantitative point of view.

The CIPSI algorithm being our practical scheme for generating selected CI expansions, let us now present its main steps.

• Step 0: Start from a given determinant (e.g. the Hartree-Fock determinant) or set of determinants, thus defining an initial reference subspace: S 0 = {|D 0 , ...}. Diagonalize H within S 0 and get the ground-state energy E (0) 0 and eigenvector:

|Ψ (0) 0 = i∈S 0 c (0) i |D i (3)
Here and in what follows, a superscript on various quantities is used to indicate the iteration number.

Then, do iteratively (n = 0, ...):

• Step 1: Collect all different determinants |D k connected by H to |Ψ (n) 0 , that is Ψ (n) 0 |H|D k = 0 (4)
and not belonging to the reference space S n .

• Step 2: Compute the small energy change of the total energy due to each connected determinant as evaluated at second-order perturbation theory

δe(|D k ) = - | Ψ (n) 0 |H|D k | 2 H kk -E (n) 0 (5)
• Step 3: Add the determinant |D k * associated with the largest |δe| to the reference subspace:

S n → S n+1 = S n ∪ {|D k * }
Of course, instead of adding only one determinant a group of determinants can be selected using a threshold. This is what is actually done in practice.

• Step 4: Diagonalize H within S n+1 to get:

|Ψ (n+1) 0 = i∈S n+1 c (n+1) i |D i with E (n+1) 0 (6)
• Go to step 1 or stop if the target size for the reference subspace has been reached.

Denoting N dets the final number of determinants, the resulting ground-state |Ψ 0 (N dets )

is the variational CIPSI solution. It is the expansion used in DMC to contruct the determinantal part of the trial wavefunction.

A second step in CIPSI is the calculation of a perturbational estimate of the correlation energy left between the variational CIPSI energy and the exact FCI one. At second order, this contribution writes

E P T 2 = - k∈M | Ψ 0 (N dets )|H|D k | 2 H kk -E 0 (N dets ) (7)
where M denotes the set of all determinants not belonging to the reference space and connected to the CIPSI expansion |Ψ 0 (N dets ) by H (single and double excitations only)

and E 0 (N dets ) the variational CIPSI energy. In practice, this contribution allows to recover a major part of the remaining correlation energy.

At this point a number of remarks are in order:

i.) Although the selection scheme is presented here for computing the ground-state eigenvector only, no special difficulties arise when generalizing the scheme to a finite number of states (see, e.g. [17])

ii.) The decomposition of the Hamiltonian H underlying the perturbative second-order expression introduced in step 2 is known as the Epstein-Nesbet partition. [18,19] This decomposition is not unique, other possible choices are the Møller-Plesset partition [20] or the barycentric one, [8] see discussion in [17].

iii.) Instead of calculating the energetic change perturbatively, expression (5), it can be preferable to employ the non-perturbative expression resulting from the diagonalization of H into the two-dimensional basis consisting of the vectors |Ψ

δe(|D k ) = 1 2 [H kk -E 0 (N dets )]    1 -1 + 4| Ψ (n) 0 |H|D k | 2 [H kk -E 0 (N dets )] 2    (8)
In the limit of small transition matrix elements, Ψ

(n) 0 |H|D k , both expressions ( 5) and ( 8) coincide. The non-perturbative formula is used in our applications. iv.) The implementation of this algorithm can be performed using limited amount of central memory. On the other hand, the CPU time required is essentially proportional to

N dets N 2 occ N 2 virt
where N occ is the number of occupied molecular orbitals and N virt the number of virtual orbitals.

C. Selected CI variants

As already pointed out selecting the most important determinants of the FCI expansion is a so natural idea that, since the pioneering work of Bender and Davidson [6] and Whitten and Hackmeyer, [7] several variants of SCI approaches have been proposed. In practice, the actual differences between approaches are usually rather minor and most ideas and technical aspects seem to have been re-discovered several times by independent groups. To give a fair account of the subject and an exhaustive list of references is thus difficult. Here, we limit ourselves to the references we are aware of, namely [6-8, 17, 21-41]. Regarding more specifically CIPSI, there has been a sustained research activity conducted during the 80's and 90's by research groups in Toulouse (Malrieu and coll.), Pisa (Angeli, Persico, Cimiraglia and coll.), and then Ferrara (Angeli, Cimiraglia) including the development at Pisa of a very efficient CIPSI code using diagrammatic techniques [START_REF] Cimiraglia | [END_REF]31,42]. Thanks to all this, CIPSI has been extensively applied for years by several groups to a variety of accurate studies of ground and excited states and potential energy surfaces (see, for example [43][44][45][46][47][48][49][50][51][52][53][54][55][56][57][58]) Finally, note that in the last years our group has developped its own CIPSI code, Quantum Package. This code has been designed to be particularly easy to install, run and modify; it can be freely downloaded at [59].

D. FCI-QMC as a stochastic selected CI approach Full Configuration Interaction Quantum Monte Carlo (FCI-QMC) is a method for solving stochastically the FCI equations. [9,10] Introducing as in DMC an imaginary time t the coefficients c i of the CI expansion, Eq.(1), are evolved in time using the operator [1 -τ (H -E)] as small-time propagator

c(t + τ ) = [1 -τ (H -E)]c(t) (9)
c being the vector of coefficients, E some reference energy, and τ the time step. After n steps the coefficients are given by

c(t) = [1 -τ (H -E)] n c(t = 0). ( 10
)
In the long-time limit (t = nτ large) the vector c converges to the exact CI vector (independently on initial conditions c(t=0) provided that c(t=0)|c = 0 and for a sufficiently small time step). As in all QMC methods, a set of walkers is introduced for sampling coefficients and a few simple stochastic rules realizing in average the action of H according to Eq.( 9) are introduced (spawning, death/cloning and annihilation). Note that equations of evolution (10) are similar to those of continuous DMC (electrons moving in ordinary space) where a small-time expression of operator e -τ (H-E) is used, and are essentially identical to the equations of lattice DMC (see e.g., [60]) The two main differences of FCIQMC with other QMC approaches are the fact that no trial vector is introduced (thus, avoiding the fixednode error) and that the stochastic rules used are particularly efficient in attenuating the sign instability inherent to all stochastic simulations of fermionic systems (annihilation at each MC step of walkers of opposite sign on occupied determinants and use of the initiator approximation).

At a given time t the CI expansion is stochastically realized by the distribution of walkers as

|Ψ = i n i |D i
where n i is the sum of the signed weight of walkers on Slater determinant

|D i (M = i |n i |=
total number of walkers). This wavefunction is the counterpart of the CIPSI expansion at iteration n, Eq.( 6). As in CIPSI at the next step t+τ (next iteration n+1) new determinants will appear. In FCI-QMC it is realized through spawning. Some determinants may also disappear through the action of the diagonal part of the Hamiltonian

[1 -τ (H ii -E)]
(death/cloning step). These two steps are designed to reproduce in average the action of the propagator on determinant

D i [1 -τ (H -E)]|D i = [1 -τ (H ii -E)]|D i -τ k =i H ik |D k .
In CIPSI a given determinant |D i is selected only once during iterations via Eq.( 5). In latter iterations it is included in the reference space and does not participate anymore to the selection. Starting from some initial determinant (usually the HF determinant) the probability of selecting |D i at some given iteration n is related to the existence of a series of (n -1)

intermediate determinants (|D i 1 , |D i 2 , ..., |D i k , .
..) different from |D i and connecting it to the initial determinant so that the product

k |H i k+1 i k | 2 H i k+1 i k -E 0
is large compared to products corresponding to other series of intermediate determinants.

In FCI-QMC a determinant |D i is spawned (selected) from |D j according to the magnitude of H ii and -in contrast with CIPSI-with no direct dependence on the inverse of (H ii -E 0 ). However, during MC iterations the number of walkers on a given determinant evolves in time according to the death/cloning step and leads to a weighted contribution of determinants to spawning. After integration in time the weight of the determinants |D i can be estimated to be about

dte -t(H ii -E 0 ) that is, ∼ 1
H
ii -E 0 for large enough time. As seen, FCI-QMC and CIPSI are in close connection.

III. APPLICATIONS OF CIPSI

A. The water molecule

To exemplify CIPSI all-electron calculations for the water molecule using basis sets of various sizes are presented. In our first example we propose to reproduce the density matrix renormalization group (DMRG) calculation of Chan and Head-Gordon [61] at geometry (R OH = 1 Å, θ OH = 104.5 • ) and using the "Roos Augmented Double Zeta ANO" basis set consisting of 41 orbitals [62,63]. The full CI Hilbert space contains about 5.6 10 11 determinants (no spin or space symmetries taken into account). Calculations have been carried out using our perturbatively selected CI program Quantum Package. [59] The energy convergence as a function of the number of selected determinants in different situations is presented in CIPSI variational energy as a function of the number of selected determinants up to 750 000 using either canonical or natural molecular orbitals. Natural orbitals have been obtained by diagonalizing the first-order density matrix built with the largest expansion obtained using canonical orbitals. As seen the convergence of both variational energies is very rapid. Using canonical orbitals an energy of -76.31239 a.u. is obtained with 750 000 determinants, a value differing from the FCI one by only 2.3 millihartree (about 1.4 kcal/mol). As known the accuracy of CI calculations is significantly enhanced when using natural orbitals. [START_REF] Davidson | Natural Orbitals[END_REF] Here, it is clearly the case and the lowest energy reached is now -76.31436 a.u. with an error of 0.35 millihartree (about 0.2 kcal/mol). When adding the second-order energy correction E P T 2 , Eq.( 7), the energy convergence is much improved (two lower curves of Figure 2).

The kcal/mol (chemical) accuracy is reached with only 1000 and 4000 determinants using canonical and natural orbitals, respectively. The best CIPSI energy including second-order correction and obtained with canonical orbitals is -76.31452 a.u. When using natural orbitals the energy is found to converge with five decimal places to the value of -76.31471 a.u., in perfect agreement with the DMRG result of Chan and Head-Gordon, -76.31471(1) a.u.

Let us emphasize that approaching the FCI limit with such a level of accuracy and so few determinants (compared to the total number of 5.6 10 11 ) is particularly striking and is one of the most remarkable features of SCI approaches.

To illustrate the possibility of making calculations with much larger basis sets, results obtained with the correlation-consistent polarized core-valence basis sets, cc-pCVnZ, with The graph on the left displays the variational energy, and the graph on the right shows the energy with the perturbative correction, Eq. (7).

gence of the ground-state variational energy obtained for each basis set is shown. As seen, the convergence is still possible with such larger basis sets. On the right part, the full CIPSI energy curves (E var + E P T 2 ) are presented; each curve is found to converge with a good accuracy to the full CI limit.

B. Generalization: The G1 set

In contrast with the exact Full-CI approach which takes into account the entire set of determinants and is thus rapidly unfeasible, CIPSI can be used for much larger systems. The exact limits depend of course on the size of the basis set used, the number of electrons, and also on the level of convergence asked for when approaching the full CI limit. To illustrate the feasibility of CIPSI for larger systems we present systematic all-electron calculations for the G1 benchmark set of Pople and collaborators. [65] The set is composed of 55 molecules and 9 different atoms. The cc-pVDZ and cc-pVTZ basis sets have been used. For all systems and both basis sets a quasi-FCI convergence has been reached. In Figure 4 the number of selected determinants needed to recover 99% of the correlation energy at CIPSI variational level (cc-pVDZ basis set) is plotted for each molecule or atom. For each system results are given either for canonical or natural orbitals. Depending on the importance of the multiconfigurational character of the system, this number may vary considerably (from a few tens to about 10 7 ). As expected, the number of determinants needed using natural orbitals is most of the times smaller and sometimes comparable. Figure 5 is similar to the preceding figure, except that numbers are given now for a full CIPSI calculation including the second-order energy correction and that a much greater accuracy corresponding to 99.9% of the correlation energy is targeted. As seen, it is remarkable that such a high precision can be reached for all systems with a number of determinants not exceeding ∼ 10 7 . In contrast with variational calculations, it should be noted that the use of natural orbitals does not systematically improve the convergence. Finally, some comparison with accurate CCSD(T) calculations performed using the same basis sets and geometries are presented. In Figure 6 the distribution of errors in atomization energies calculated with both CCSD(T) and CIPSI methods are plotted. For the cc-pVDZ basis set, CCSD(T) and CIPSI curves are very similar, indicating that CCSD(T) calculations have also reached the quasi full CI limit. For the larger cc-pVTZ basis set, the two curves remain similar but some significant differences show up with CIPSI results more distributed toward small errors due to a better description of multireference systems.

BeH

IV. USING CIPSI NODES IN DMC

A. Motivations

In DMC the standard practice is to introduce compact trial wavefunctions reproducing as much as possible the mathematical and physical properties of the exact wave function.

Next, the "best" nodes are determined through optimization of the parameters of the trial wavefunction in a preliminary variational Monte Carlo (VMC) run. The objective function to minimize is either the variational energy associated with the trial wavefunction or the variance of the Hamiltonian (or a combination of both). A number of algorithms have been elaborated to perform this important practical step as efficiently as possible. [66][67][68][69][70][71] No limitations existing in QMC for the choice of the functional form of the trial wavefunction, many different expressions have been introduced (see, e.g. [72][73][74][75][76][77][78][79]). However, the most popular one is certainly the Jastrow-Slater trial wavefunction expressed as a short expansion over a set of Slater determinants multiplied by a global Jastrow factor describing explicitly the electron-electron and electron-electron-nucleus interactions and, in particular, imposing the electron-electron cusp conditions associated with the zero-interelectronic distance limit of the true wavefunction.

In the DMC-CIPSI approach the determinantal part of the trial wavefunction is built using systematic CIPSI expansions. The main motivation is that CI approaches provide a simple, deterministic, and systematic way of constructing wavefunctions of controllable quality. In a given one-particle basis set, the wavefunction is improved by increasing the number of determinants, up to the Full CI (FCI) limit. Then, by increasing the basis set, the wavefunction can be further improved, up to the complete basis set (CBS) limit

where the exact solution of the continuous electronic Schrödinger equation is reached. The CI nodes, which are defined as the zeroes of the expansion, are also expected to follow such a systematic improvement, thus facilitating the control of the fixed-node error. A second important motivation is that the stochastic optimization step can be avoided since a systematic way of improving the wavefunction is now at our disposal. The optimal CI coefficients are obtained by the (deterministic) diagonalization of the Hamiltonian matrix in the basis set of Slater determinants. It is a simple and robust step which leads to a unique set of coefficients. Furthermore, it can be made automatic, an important feature in the perspective of designing a fully black-box QMC code. Finally, using deterministically constructed nodal structures greatly facilitates the use of nodes evolving smoothly as a function of any parameter of the Hamiltonian. It is important when calculating potential energy surfaces (see, our application to the F 2 molecule, [4]) or response properties under external fields.

The main price to pay for such advantages is of course the need of considering much larger multideterminant expansions (from tens of thousands up to a few millions) than in standard DMC implementations where compactness of the trial wavefunction is searched for. However, efficient algorithms have been proposed to perform such calculations [80][81][82].

Very recently, we have also presented an efficient algorithm for computing very large CI expansions. Its main ideas are briefly summarized in section IV C below.

B. Toward a better control of the fixed-node approximation

A remarkable property systematically observed so far in our DMC applications using large CIPSI expansions [1,[START_REF] Scemama | [END_REF]4] is that, except for a possible transient regime at small number of determinants,[83] the fixed-node error resulting from the use of CIPSI nodes is found to decrease monotonically, both as a function of the number of selected determinants, N dets , and of the basis set size, M . This result is illustrated here in the case of the water molecule at equilibrium geometry. Results shown here complement our recent benchmark study on water. [5]. In Figure 7 all-electron fixed-node energies obtained with DMC-CIPSI as a function of the number of selected determinants for the first four cc-pCVnZ basis set (n=2-5) are reported. Calculations have been performed using the variational CIPSI expansions of the preceding subsection. In practice, DMC simulations have been realized using our generalpurpose QMC program QMC=Chem (downloadable at [START_REF] Scemama | Qmc=chem[END_REF]). A minimal Jastrow prefactor taking care of the electron-electron cusp condition is employed and molecular orbitals are slightly modified at very short electron-nucleus distances to impose exact electron-nucleus cusp conditions. The time step used, τ = 2 × 10 -4 a.u., has been chosen small enough to make the finite time step error not observable with statistical fluctuations. As seen on the figure the convergence of DMC energies both as a function of the number of determinants and of the basis set are almost reached. The value of -76.43744(18) a.u. obtained with the largest basis set and 1 423 377 determinants is, to the best of our knowledge, the lowest upper bound reported so far, the experimentally derived estimate of the exact nonrelativistic energy being -76.4389(1) a.u. [START_REF] Klopper | [END_REF] Thanks to our recent algorithm for calculating very large number of determinants in DMC [13] (see, section IV C below), the increase of CPU time for the largest calculation including more than 1.4 million of determinants compared to the same calculation limited to the Hartree-Fock determinant is only ∼ 235.

In practice, the possibility of calculating fixed-node energies displaying such a regular behavior as a function of the number of determinants and molecular orbitals is clearly attractive in terms of control of the fixed-node error. For example, in our benchmark study of the water molecule [5] it was possible to extrapolate the DMC energies obtained with each cc-pCVnZ basis set as a function of the cardinal number n, as routinely done in deterministic CI calculations. Using a standard 1/n 3 law a very accurate DMC-CIPSI energy value of -76.43894(12) a.u. was obtained, in full agreement with the estimate exact value of -76.4389(1) a.u. [5] At this point, we emphasize that the observed property of systematic decrease of the energy as a function of the number of determinants is known not to be systematically true for a general CI expansion (see, e.g. [START_REF] Flad | Recent Advances in Quantum Monte Carlo Methods[END_REF]). Here, its validity may probably be attributed to the fact that determinants are selected in a hierarchical way (the most important ones first), so that the wavefunctions quality increases step by step, and so the quality of nodes. However, from a mathematical point of view, such a property is far from being trivial. There is no simple argument why the FCI nodes obtained from minimization of the variational energy with respect to the multideterminant coefficients would lead to the best nodal structure (minimum of the fixed-node energy with respect to such coefficients). In a general space (not necessarily a Hilbert space of determinants) it is easy to construct a wavefunction of poor quality having a high variational energy but exact nodes and, then, to exhibit a wavefunction with a much lower energy but wrong nodes. To demonstrate the validity or not of the observed property in a finite space of determinants built with molecular orbitals expanded in a finite basis set remains to be done.

C. Evaluating very large number of determinants in QMC

The algorithm we use to run DMC calculations with a very large number of determinants (presently up to a few millions) has been presented in detail in [13]. Its efficiency is sufficiently high to perform converged DMC calculations with a number of determinants up to a few millions of determinants. In the case of the chlorine atom discussed in [13] a trial wavefunction including about 750 000 determinants has been used with a computational increase of about 400 compared to a single-determinant calculation. As already mentioned above, in the benchmark calculation of the water molecule [5] up to 1 423 377 determinants have been used for a computational increase of only ∼ 235.

The main ideas of the algorithm are as follows.

• spin-specific determinants

D σ i (R σ ) (σ =↑, ↓) as follows Ψ Det (R) = N ↑ dets i=1 N ↓ dets j=1 C ij D ↑ i (R ↑ )D ↓ j (R ↓ ) ( 11
)
where C is a matrix of coefficients of size N ↑ dets × N ↓ dets , R = (r 1 , ..., r N ) denotes the full set of electron space coordinates, and R ↑ and R ↓ the two subsets of coordinates associated with ↑ and ↓ electrons.

In standard CI expansions the number of unique spin-specific determinants is much smaller than N dets and typically scales as √ N dets . It is true for FCI expansions where all possible determinants are considered. Indeed, N σ dets attains its maximal value of N MO Nσ and since N dets is given as N ↑ dets × N ↓ dets , the number of unique spin-specific determinants D σ (R) is of order √ N dets . However, it is in general also true for the usual truncated expansions (CASSCF, CISD, etc.) essentially because the numerous excitations implying multiple excitations of spin-like electrons plays a marginal role and have a vanishing weight.

• Optimized Sherman-Morrison updates. As proposed in a number of works, [80][81][82] we calculate the determinants and their derivatives using the Sherman-Morrison (SM) formula for updating the inverse Slater matrices. However, in contrast with other implementations, we have found more efficient not to compare the Slater matrix to a common reference (typically, the Hartree-Fock determinant) but instead to perform the Sherman-Morrison updates with respect to the previously computed determinant D σ i-1 . To reduce the prefactor associated with this step the list of determinants is sorted with a suitably chosen order so that with high probability successive determinants in the list differ only by one-or two-column substitution, thus decreasing the average number of substitution performed.

• Exploiting high-performance capabilities of present-day processors. This very practical aspect -which is in general too much underestimated -is far from being anecdotal since it allows us to gain important computational savings. A number of important features include the use of vector fused-multiply add (FMA) instructions (that is, the calculation of a=a+b*c in one CPU cycle) for the innermost loops. It is extremely efficient and should be systematically searched for. Using such instructions (present in general-purpose processors), up to eight FMA per CPU cycle can be performed in double precision. While computing loops, overheads are also very costly and should be reduced/eliminated. By taking care separately of the various parts of the loop (peeling loop, scalar loop, vector loop, and tail loop) through size-specific and/or hard-coded subroutines, a level of 100% vectorized loops is reached in our code. Another crucial point is to properly manage the data flow arriving to the processing unit. As known, to be able to move data from the memory to the CPU with a sufficiently high data transfer to keep the CPU busy is a major concern of modern calculations. Then, it is not only important to make maximum use of the low-latency cache memories to store intermediate data but also to maximize prefetching allowing the processor to anticipate the use of the right data and instructions in advance. To enhance prefetching the algorithm should allow the predictability of the data arrival in the CPU (that is, avoid random access as much as possible). It is this important aspect that has motivated us to use Sherman-Morrison updates, despite the fact that a method like the Table method [81] has formally a better scaling. Indeed, massive calculations of scalar products at the heart of repeated uses of SM updates are so ideally adapted to present-day processors that very high performances can be obtained.

• Improved truncation scheme. Instead of truncating the CI expansion according to the magnitude of the multideterminant coefficients as usual done, we propose instead to remove spin-specific determinants according to their total contribution to the norm of the expansion. In this way, more determinants can be handled for a price corresponding to shorter expansions.

To be more precise, we first observe that truncating the wavefunction according to the magnitude of coefficients has the effect of removing elements of the sparse matrix C of Eq. (11). A reduction of the computational cost occurs only when a full line (↑) or a full column (↓) of C contains only zeroes, in that case the determinant D σ can be removed from the calculation. Now, by expressing the norm of the wave function as

N = N ↑ dets i=1 N ↓ dets j=1 C 2 ij = N ↑ dets i=1 N ↑ i = N ↓ dets j=1 N ↓ j . (12)
it is possible to assign a contribution to the norm to each determinant. Then, all determinants whose contribution to the norm is below some threshold will be removed from the expansion. This truncation scheme allows to eliminate the smallest number of coefficients needed to obtain some computational gain. Moreover, the size-consistence property of the wave function is expected to be approximately preserved by such a truncation : when a σ-determinant is removed, it is equivalent to removing the product of D σ with all the σ-determinants of the wave function.

V. PSEUDOPOTENTIALS FOR DMC USING CIPSI

When using pseudopotentials a valence Hamiltonian is defined

H val = H loc + V ECP (13)
where H loc is the local part describing the kinetic energy, the Coulombic repulsion and the local part of the effective core potentials (ECP).

H loc = - 1 2 i ∇ 2 i + i,α v loc (r iα ) + i<j 1 r ij (14)
and V ECP the non-local part written as

V ECP = i,α l v l (r iα ) l m=-l Y lm (Ω iα ) dΩ iα Y * lm (Ω iα ) (15)
where v l is a radial pseudopotential, Y lm is the spherical harmonic, α labels pseudo-ions.

The action of a non-local operator being difficult to sample in DMC, V ECP is "localized" by projecting it on the trial wavefunction. The localized form of the pseudo-potential is thus defined as

V loc ECP = V ECP Ψ T Ψ T (16)
and we are led back to standard DMC simulations using only local operators at the price of introducing a new "localization approximation". This error is usually minimized by optimization of the trial wavefunction, see ref. [START_REF] Casula | [END_REF]. In practice, the necessity of numerically evaluating the localized potential is the main difference with standard DMC calculations.

For each nucleus α and electron i, the two-dimensional angular integrals of the product of each Y lm and the trial wavefunction (all electrons fixed except the ith-electron moved over the sphere centered on nucleus α and of radius r iα ) must be performed. By choosing the axes oriented such that the ith electron is on the z axis, the contribution coming from the pair (i, α) is given by [16] i,α l

2l + 1 4π v l (r iα ) dΩ iα P l (cosθ ) ψ T (r 1 , ...r i , ..., r N ) ψ T (r 1 , ..r i , ..r N ) (17)
where P l denotes a Legendre polynomial. Because of the Jastrow factor, the integrals involved cannot be computed analytically. The standard solution is to evaluate them numerically using some quadrature for the sphere. Here, the CI form allows to perform the integration exactly, as already proposed some time ago. [14,15] Note that although no Jastrow prefactor is used here when localizing the pseudo-potential operator, such a prefactor can still be used for the DMC simulation itself. A first advantage is that the calculation is significantly faster: in practice, the computational cost is the same as evaluating the Laplacian of the wave function and a gain proportional to the number of quadrature points is obtained. A second advantage is the possibility of a better control of the localization error by increasing the number of determinants.

To illustrate these statements, we have chosen to calculate the atomization energy of the C 2 molecule at the Hartree-Fock, CIPSI, DMC-HF and DMC-CIPSI levels with and without pseudopotentials. All-electron HF or CIPSI calculations have been performed with the cc-pVTZ basis set. To allow meaningful comparisons, 1s molecular orbitals have been kept

frozen in all-electron CIPSI calculations. Pseudopotential calculations were done using the pseudopotentials of Burkatzki et al. [88] with the corresponding VTZ basis set. The electronnucleus cusps of all the wave functions were imposed, [89][90][91] and no Jastrow factor was used.

For the sake of comparison, the same time step (5 × 10 -4 au) was used for all-electron and pseudopotential calculations, although a much larger time step could have been taken with pseudopotentials. The results presented in Table I show that all the atomization energies obtained using pseudopotentials are in very good agreement with those obtained with all-electron calculations at the same level of theory. The DMC energies obtained with CIPSI trial wave functions are always below those obtained with Hartree-Fock trial wave functions, and the error in the atomization energy is reduced from 20 kcal/mol with HF nodes down to 3 kcal/mol with CIPSI nodes.

Calculations were performed on Intel Xeon E5-2680v3 processors. Timings are given in Table II. For the carbon atom the computational time needed for one walker to perform one complete Monte Carlo step (all electrons moved) is the same with or without pseudopotentials. For the C 2 molecule, the calculation is even faster with pseudopotentials: A factor of about 1.5 is gained with respect to the all-electron calculation. This can be explained by the computational effort saved due to the reduced size of Slater matrices in the pseudopotential case (from 6 × 6 to 4 × 4) but, more importantly, by the fact that the additional cost related to the calculation of the contributions due to the pseudopotential is not enough important to reverse the situation. In all-electron calculations, the variance is only slightly reduced when going from the Hartree-Fock trial wave function to the CIPSI wave function (with frozen core). Indeed, the largest part of the fluctuations comes from the lack of correlation of the core electrons. In the calculations involving pseudopotentials, the decrease of the variance is significant: a reduction by a factor of 2.4 and 3.2 is observed.

From a more general perspective, comparisons between all-electron and pseudopotential calculations must take into account both the computational effort required in each case and the level of fluctuations resulting from the quality of the trial wavefunction. fraction of the full determinantal space. This is so because only the most important determinants of the FCI expansion are perturbatively selected at each step of the iterative process.

We note that the recent FCI-QMC method of Alavi et al. [9,10] uses essentially the same idea, except that in CIPSI the selection is done deterministically instead of stochastically.

ii.) In constrast with exact FCI which becomes rapidly prohibitively expensive, CIPSI allows to treat larger systems, while maintaining results of near-Full CI quality. The exact practical limits depend of course on the size of the basis set used, the number of active electrons, and also on the level of convergence asked for when approaching the full CI limit.

In this work, the CIPSI approach has been exemplified with near-FCI quality all-electron calculations for the water molecule using a series of basis sets of increasing size up to the cc-pCV5Z basis set and for the whole set of 55 molecules and 9 atoms of the benchmark G1 set (cc-pVDZ basis set). In each case, the huge size of the FCI space forbids exact FCI calculations. CIPSI has been applied to larger systems, for example for calculating accurate total energies for the atoms of the 3d series, [START_REF] Scemama | [END_REF] and for obtaining near-FCI quality results for the CuCl 2 molecule (calculations including 63 electrons and 25 active valence electrons). [94] Note that by using Effective Core Potentials as described in section V even larger systems can be treated.

iii.) We emphasize that the idea of selecting determinants is not limited to the entire space of determinants but can be used to make CI expansion to converge in a subset of determinants chosen a priori. For example, efficient and accurate selected CASCI, CISD, or even MRCC [95] calculations can be performed. Note that going beyond CASCI and implementing a selected CASSCF approach (CASCI with optimization of molecular orbitals) is also possible; this is let for further work. However, note that a stochastic version of CASSCF within FCI-QMC framework has already been implemented by Alavi et al. [96] iv.) CIPSI expansions can be used as determinantal part of the trial wavefunctions employed in DMC calculations. In others words, we propose to use selected CI nodes as approximation of the unknown exact nodes. The basic motivation is that CI approaches provide a simple, deterministic, and systematic way to build wavefunctions of controllable quality. In a given one-particle basis set, the wavefunction is improved by increasing the number of determinants, up to the FCI limit. Then, by increasing the basis set, the wavefunction can be further improved, up to the CBS limit where the exact solution of the continuous electronic Schrödinger equation is reached. CI nodes, defined as the zeroes of the CI expansions, are also expected to display such a systematic improvement.

v.) The main result giving substance to the use of selected CIPSI nodes is that in all applications realized so far the fixed-node error is found to decrease both as a function of the number of selected determinants and of the size of the basis set. Mathematically speaking, such a result is far from being trivial. In practice, such a property is particularly useful in terms of control of the fixed-node error.

vi.) From a practical point of view, the price to pay is the need of considering much larger multideterminant expansions (from tens of thousands up to a few millions) than in standard DMC where compactness of the trial wavefunction is usually searched for. Indeed, computing at each of Monte Carlo step the first and second derivatives of the trial wavefunction (drift vector and local energy) is the hot spot of DMC. However, efficient algorithms have been proposed to perform such calculations [80][81][82]. Here, we have briefly summarized our recently introduced algorithm allowing to compute N -determinant expansions issued from selected CI calculations with a computational cost roughly proportional to √ N (with a small prefactor).

vii.) One key advantage of using CIPSI nodes is that their construction can be made fully automatic. Coefficients of the CI expansion are obtained in a simple and deterministic way by diagonalizing the Hamiltonian matrix and the solution is unique. Furthermore, when approaching the FCI limit the resulting expansion becomes independent on the type of molecular orbitals used (canonical, natural, Kohn-Sham, see Figure 8 of ref. [94]). Another attractive feature is that the nodes built are reproducible and thus "DMC models" can be defined in the spirit of WFT or DFT ab initio approaches (HF/cc-pVnZ, MP2/6-31G, CCSD(T), DFT/B3LYP etc.) Indeed, once the basis set has been specified, the nodes are unambiguously defined at convergence of the DMC energy as a function of the number of selected determinants. Furthermore, in this limit the nodal surfaces vary continuously as a function of the parameters of the Hamiltonian. A particularly important example is the possibility of obtaining regular potential energy surface (PES). This idea has been illustrated in a previous work on the potential energy curve of the F 2 molecule. [4] Furthermore, it is also possible to reduce the "non-parallelism" error resulting from the use of a trial wavefunction of non-uniform quality across the PES. This can be done for example by using a variable number of selected determinants depending on the geometry and chosen to lead to a constant second-order estimate of the remaining correlation energy (constant-PT2 approach, [4] ).

viii.) As in standard DMC approaches a Jastrow prefactor can be used to reduce statistical fluctuations. However, in contrast with what is usually done, we do not propose to re-optimize the determinantal CIPSI part in presence of this Jastrow term. The main reason for that is not to lose the advantages of using deterministically constructed nodal structures: Systematic improvement of nodes as a function of the number of determinants and of the size of the basis set, simplicity of construction of nodes and reproductibility, possibility of optimizing a very large number of small coefficients in the CI expansion (no noise limiting in practice the magnitude of optimizable coefficients), smooth evolution of nodes under variation of an external parameter (geometry, external field), etc.

ix.) The price to pay for not re-optimizing the determinantal part in the presence of a Jastrow is that for small basis sets larger fixed-node errors are usually obtained. However, when increasing sufficiently the quality of basis set, it is no longer true as illustrated for example in the case of the oxygen atom, [1] the water molecule, [5] and the 3d-transition metal atoms [START_REF] Scemama | [END_REF] for which benchmark total energies have been obtained.

x.) CIPSI wavefunctions are particularly attractive when using non-local Effective Core Potentials (ECP). Indeed, as already proposed some time ago, [14,15] CI expansions allow the analytical calculation of the action of the non-linear part of the pseudo-potential operator on the trial wavefunction. In this way, the use of a numerical grid defined over the sphere is avoided and a gain in computational effort essentially proportional to the number of grid points is obtained. Here, this idea has been illustrated in the case of the C 2 molecule.

Finally, let us briefly mention a number of topics presently under investigation.

xi.) The slow part of the CI convergence is known to result from the absence of electronelectron cusp. In standard QMC approaches, the short distance electron-electron behavior is introduced into the Jastrow prefactor and its impact on nodes is taken into account by optimization of the full trial wavefunction. Under re-optimization, molecular orbitals are changed and the distribution of multideterminant coefficients is modified with a reinforcement of coefficients associated with chemically meaningful determinants and a reduction of the numerous small coefficients associated with the absence of cusp. To keep the CIPSI as compact as possible and to eliminate this unphysical and uncoherent background of small coefficients a R12/F12 version of CIPSI is called for. We emphasize that such an analytical and deterministic construction of the R12/F12 expansion is necessary if we want to keep the advantages related to the deterministic construction of nodes.

xii.) To treat even larger systems, the increase of the number of determinants in the CIPSI expansion must be kept under control. Instead of targeting the near full CI limit, simpler models can be used in the spirit of what is done in MRCC approaches [95] or by defining effective Hamiltonians in the reference space modelling the effect of the external space (so-called internally decontracted approaches).

xiii.) Finally, it is clear that systematic studies on difficult systems of various types are needed to explore the potential and limits of the DMC-CIPSI approach.

Figure 1 . N 2

 12 Figure 1. N 2 in the cc-pVTZ basis set (R N -N =1.0977 Å). Variation of the number of determinants with n-excitations with respect to the Hartree-Fock determinant in the CIPSI expansion as a function of the number of selected determinants up to 5 × 10 6 .

  presents the number of determinants per class of excitations n as a function of the number of determinants in the CIPSI wavefunction for the N 2 molecule at equilibrium geometry (cc-pVTZ basis set). Without entering now into the details of CIPSI presented below, let us just note that for 5×10 6 determinants the CIPSI expansion has almost converged to the FCI solution. Accordingly, results presented in the figure for the distribution of excitations is essentially that of the FCI wavefunction.

and

  |D k . Simple algebra shows that the energetic change is given by

Figure 2 .

 2 Figure 2. Four different curves are shown together with the DMRG energy value of -76.31471(1) of Chan and Head-Gordon[61] (solid horizontal line). The two upper curves represent the

Figure 2 .

 2 Figure 2. Energy convergence of the variational and full CIPSI energies as a function of the number of selected determinants using canonical and natural orbitals. Energy in a.u.

n

  going from 2 to 5 are presented. The geometry chosen is now the experimental equilibrium geometry, R OH = 0.9572 Å and θ OH = 104.52 • . The number of basis set functions are 28, 71, 174 and 255 for cc-pCVDZ, cc-pCVTZ,cc-pCVQZ, and cc-pCV5Z, respectively. The total number of determinants of the FCI Hilbert space with such basis sets are about 1 10 10 ,1.7 10 14 ,1.6 10 18 , and 7.5 10 19 , respectively. On the left part of Figure 3 the conver-E(var)+PT2 with natural MOs cc-pCVDZ cc-pCVTZ cc-pCVQZ cc-pCV5Z

Figure 3 .

 3 Figure 3. Convergence of the energy with the number of selected determinants (logarithmic scale).

Figure 5 .Figure 6 .

 56 Figure 5. Number of selected determinants required to recover 99.9% of the total correlation energy at full CIPSI/cc-pVDZ level (E var + E P T 2 ). Results for canonical and natural orbitals are given.

Figure 7 .

 7 Figure 7. DMC energy of the water molecule as a function of the number of determinants in the trial wave function (logarithmic scale). The horizontal solid line indicates the experimentally derived estimate of the exact nonrelativistic energy.[START_REF] Klopper | [END_REF]

Table II .

 II To quantify this, we have reported in the table the number of CPU hours required to obtain an error bar of 1 kcal/mol. Using pseudopotentials for the C 2 molecule, it is found that the reduction of the variance due to the improvement of the wave function with the multideterminant expansion almost compensates the cost of the computation due to the additional 230 000 determinants : the CPU time needed to obtain a desired accuracy is only 1.2× more than the single determinant calculation.Let us first summarize the most important ideas and results presented in this work. i.) Selected Configuration Interaction approaches such as CIPSI are very efficient methods for approaching the full CI limit with a number of determinants representing only a tiny CPU time for one complete Monte Carlo step (one walker, all electrons moved), CPU time needed to reach an error on 1 kcal/mol, and variances associated with the HF and CIPSI trial wave functions (electron-nucleus cusp corrected).

		CPU time per DMC step CPU time to get a 1 kcal/mol	Variance
			(milliseconds)	error (hours)		(a.u.)
		all-e	pseudo-	all-e	pseudo-	all-e	pseudo-
	DMC-HF
	C	0.0076	0.0078	1.54	1.18	7.858(3) 0.3471(2)
	C 2	0.0286	0.0186	14.95	10.35	16.208(7) 1.1372(6)
	DMC-CIPSI
	C	0.193	0.201	5.61	0.70	7.620(8) 0.1084(4)
	C 2	10.1	8.12	91.05	12.72	15.61(3) 0.460(1)
	VI. SUMMARY AND SOME PERSPECTIVES

Acknowledgments. We would like to thank C. Angeli and P-F. Loos for their useful comments on the manuscript. AS and MC thank the Agence Nationale pour la Recherche (ANR) for support through Grant No ANR 2011 BS08 004 01. This work was performed using HPC resources from CALMIP (Toulouse) under allocation 2016-0510 and from GENCI-TGCC (Grant 2016-08s015).


All-electron Fixed-node Diffusion Monte Carlo calculations for the nonrelativistic ground-state energy of the water molecule at equilibrium geometry are presented. The determinantal part of the trial wavefunction is obtained from a selected Configuration Interaction calculation [Configuration Interaction using a Perturbative Selection done Iteratively (CIPSI) method] including up to about 1.4 × 10 6 of determinants. Calculations are made using the cc-pCVnZ family of basis sets, with n = 2 to 5. In contrast with most quantum Monte Carlo works no re-optimization of the determinantal part in presence of a Jastrow is performed. For the largest cc-pCV5Z basis set the lowest upper bound for the ground-state energy reported so far of -76.437 44 (18) is obtained. The fixed-node energy is found to decrease regularly as a function of the cardinal number n and the Complete Basis Set limit associated with exact nodes is easily extracted. The resulting energy of -76.438 94 (12) -in perfect agreement with the best experimentally derived value -is the most accurate theoretical estimate reported so far. We emphasize that employing selected configuration interaction nodes of increasing quality in a given family of basis sets may represent a simple, deterministic, reproducible, and systematic way of controlling the fixed-node error in diffusion Monte Carlo. Published by AIP Publishing. [http://dx.doi.org/10.1063/1.4947093] The only uncontrolled source of error 1 in quantum Monte Carlo (QMC) methods is the fixed-node approximation introduced to suppress the wild fluctuations of the sign of the wavefunction (fermion sign problem). Although the fixednode error is small (typically, a few percents of the correlation energy), and many fixed-node QMC calculations of impressive accuracy have been realized, the error can still be too large in some applications, particularly in the important case of the computation of (very) small energy differences.

A major challenge for QMC is thus to set up a strategy of construction of trial wavefunctions having "good" nodes and, even more importantly, to propose a systematic way of improving such nodes. In practice, a standard strategy consists in introducing trial wavefunctions of the best possible quality and then to optimize their parameters in a preliminary Variational Monte Carlo (VMC) step through minimization of the variational energy or its variance. 2 Many functional forms for the trial function Ψ T have been explored in the literature, the most popular one being the Jastrow Slater form 3

Ψ T = e J  i c i D i , (1)
combining a Jastrow prefactor e J containing explicit electronic correlations and a short multi-determinantal expansion (typically, a few thousands of determinants) describing the multireference character of the wavefunction (static correlation effects). Very recently some of us have proposed to keep the standard Jastrow Slater form for the trial wavefunction but to rely on the more conventional Configuration Interaction (CI) expansions of quantum chemistry for the multideterminantal part. No stochastic re-optimization of the CI expansion is performed, so that "pure CI" nodes are employed. [4][5][6] The rationale behind this proposal is to search for a better control of the fixed-node error by exploiting the unique properties of CI wavefunctions. Indeed, CI approaches provide a simple, deterministic, and systematic way to build wavefunctions of controllable quality. In a given one-particle basis set, the wavefunction is improved by increasing the number of determinants, up to the Full CI (FCI) limit. Then, by increasing the basis set, the wavefunction can be further improved, up to the complete basis set (CBS) limit where the exact solution of the electronic Schrödinger equation is reached. CI nodes, defined as the zeroes of the CI expansions, are also expected to display such a systematic improvement. The main difficulty is of course the exponential growth of the space of determinants with respect to the number of electrons and orbitals. However, this severe exponential increase can be dramatically attenuated by considering Selected CI (SCI) approaches designed to keep only the most important determinants. In practice, we have proposed to make use of the CIPSI method (Configuration Interaction using a Perturbative Selection done Iteratively), 7,8 one of the numerous variants of SCI proposed in the literature (see, e.g., Refs. 7-15). In this approach the multideterminant expansion is built iteratively by selecting determinants according to the importance of their second-order perturbational contribution to the total energy. As illustrated by a number of applications, CIPSI represents a very efficient way of approaching the FCI limit using only a tiny fraction of the total FCI space (see, for example, a recent all-electron FCI-converged CIPSI calculation for CuCl 2 involving 25 electrons and 36 active orbitals for a FCI space including 10 18 determinants 16 ). This remarkable result is actually common to all variants of SCI approaches, including the FCI-QMC approach of Alavi et al., 17,18 which can be considered as a stochastic version of SCI. In practice, the main difficulty in using lengthy multideterminant expansions in QMC is the expensive cost of evaluating at each Monte Carlo step the first and second derivatives of the trial wavefunction (drift vector and local energy). However, efficient algorithms have been proposed to perform such calculations. [19][20][21] Here, we shall use our recently introduced algorithm allowing to perform converged diffusion Monte Carlo (DMC) calculations using multideterminant expansions including up to a few millions of determinants for a system like the water molecule. [START_REF] Scemama | Quantum Monte Carlo with very large multideterminant wavefunctions[END_REF] A remarkable property systematically observed so far in our first DMC applications using large CIPSI expansions [4][5][6] is that, except for a possible transient regime at small number of determinants, 23 the fixed-node error associated with CIPSI nodes decreases monotonically, both as a function of the number of determinants and of the basis set size, leading to the possibility of a control of the fixed-node error. Such a result is known not to be systematically true for a general CI expansion (see, e.g., Ref. 24). However, its validity here could be attributed to the fact that determinants are selected in a hierarchical way (the most important ones first), so that the wavefunction quality increases step by step, and so the quality of nodes.

In this Communication all-electron DMC/CIPSI calculations for the water molecule at equilibrium geometry using the cc-pCVnZ family of basis sets with n ranging from 2-5 and large multideterminant expansions including up to 1 423 377 determinants are presented. The lowest (upper bound) fixed-node energy reported so far of -76.437 44(18) is obtained. Performing the Complete Basis set (CBS) limit by extrapolating fixed-node energies as a function of the cardinal number n of the basis set a value of -76.438 94 (12) for the total energy associated with exact nodes is obtained, in full agreement with the best known estimate of -76.4389. [START_REF] Klopper | [END_REF] CIPSI expansion. The multideterminant CIPSI expansion is built by selecting iteratively the most important determinants of the FCI expansion. In short (for more details, see Ref. 4), at iteration n the multideterminant expansion Ψ D is written as the sum of the N (n) previously selected determinants (thus, defining the reference space at this iteration)

Ψ (n) D = N (n)  i=1 c (n) i D i , (2)
with energy

E (n) 0 = ⟨Ψ (n) D |H |Ψ (n) D ⟩ ⟨Ψ (n) D |Ψ (n) D ⟩
. Then, one determinant (or a group of determinants) D j not belonging to the reference space and corresponding to the greatest second-order energy change (or close to it within some threshold),

δE = - ⟨Ψ (n) D |H |D j ⟩ 2 ⟨D j |H |D j ⟩ -E (n) 0 , (3)
is (are) selected and added to the reference space. At iteration (n + 1) the new expansion Ψ (n+1)

D

and energy E (n+1) 0 are obtained by diagonalizing the Hamiltonian matrix within the new set of selected determinants. The iterative process is started with the Hartree-Fock determinant or a short expansion and is stopped when a target number of determinants is reached. In what follows the variational energy associated with the final CI expansion will be denoted as E var 0 . Water molecule. In this study we present benchmark calculations for the non-relativistic ground-state energy of the water molecule at equilibrium geometry, R OH = 0.9572 Å and θ OH = 104.52 • .

CIPSI results. All configuration interaction calculations have been carried out using our perturbatively selected CI program QUANTUM PACKAGE (downloadable at Ref. 26). Standard Dunning type correlation-consistent polarized corevalence basis sets cc-pCVnZ with n going from 2-5 are employed. CIPSI calculations have been performed using natural orbitals issued from the diagonalization of the onebody density matrix obtained in a preliminary CIPSI run. For each basis set, the selected CI expansion has been stopped for one million determinants, except for the largest cc-pCV5Z basis sets for which two million determinants were considered. Results are presented in Table I and compared to the recent benchmark CI calculations of Almora-Dìaz including up to sextuple excitations. 27 As we shall see below, truncated versions of these one-and two million-determinant CIPSI expansions will actually be used in DMC, results are thus presented for these shorter expansions. A remarkable point is the high efficiency of CIPSI in obtaining accurate CI expansion with a small number of determinants. For the cc-pCVDZ basis set, the variational energy obtained with the 172 256 determinants used in DMC is different from the FCI value of Almora-Dìaz by only 0.7 mhartree. For the other basis sets, the differences remain small, that is 1.8, 1.8, and 2.5 mhartree for the cc-pCVTZ, cc-pCVQZ, and cc-pCV5Z basis sets, respectively.

Fixed-node DMC (FN-DMC) results. All-electron DMC calculations have been realized using our general-purpose QMC program QMC=CHEM (downloadable at Ref. 28). A minimal Jastrow prefactor taking care of the electronelectron cusp condition is employed and molecular orbitals are slightly modified at very short electron-nucleus distances to impose exact electron-nucleus cusp conditions. The time step used, τ = 2.10 -4 a.u., has been chosen small enough to make the finite time step error not observable with statistical fluctuations.

To accelerate DMC calculations and not to use the full one-and two million-determinant expansion of the initial CIPSI calculations we have employed the improved truncation scheme described in Ref. 22. In short, the approach consists in writing the CI expansion as

N ↑ det  i=1 N ↓ det  j=1 C i j D ↑ i (R ↑ )D ↓ j (R ↓ ).
For each σ-determinant D σ k (σ =↑,↓), the contribution to the norm of the wavefunction is given either by

N ↓ det l=1 C 2 k l for σ =↑ or N ↑ det l=1 C 2 l k for σ =↓. If this contribution is below a given threshold ϵ σ the σ-determinant D σ k is discarded (C k l or C l k = 0 ∀l,
for the ↑or ↓-sector, respectively). Here, N σ det denotes the number of different σ-determinants in the CI expansion. Such numbers being usually much smaller than the total number of products of determinants D(R) = D ↑ D ↓ , the gain in computational cost can be important (see, Table 5 of Ref. 22). Here, we chose to truncate the expansion by taking ϵ ↑ = ϵ ↓ = 10 -8 , except for the cc-pCV5Z basis where a value of 10 -9 has been used. Values for ϵ σ have been chosen small enough to get converged fixed-node energies as a function of the number of selected determinants within statistical errors. In other words, nodes employed in this work are expected to be close to FCI nodes. The final numbers of selected determinants used are given in Table I.

The efficiency of our algorithm for computing large multideterminant expansions can be quantified by measuring the ratio of CPU times needed to realize one Monte Carlo step using either the full expansion or only the single HF determinant. Such ratios are presented in Table II for each basis set. Fixed-node DMC energies (in atomic units) obtained with CIPSI nodes for the various basis sets are also given in Table II and plotted in Fig. 1 as a function of the inverse of the cardinal number n = 2 to 5. The horizontal line is the best estimate of the total nonrelativistic energy reported in the literature, see Ref. 25. For comparison, we have also reported the best estimates of the FCI energies of Almora-Dìaz. Quite remarkably both sets of points display a very similar overall behavior. In particular, the values converge smoothly to the same CBS limit as a function of the cardinal number n with a typical inverse third power law. Using a simple two-parameter fitting function, E 0 (n) = E 0 (CBS) + an -3 , the CBS limit for DMC results gives an extrapolated value of -76.438 94 (12).

The error bar has been estimated by reproducing the fit over a large statistical ensemble of independent data drawn according to their respective error bars. No correlation between data being considered, the error value should be considered as rather conservative. Note that the energy of -76.437 44 ± 0.000 18 obtained with the cc-pCV5Z nodes is the lowest upper bound reported so far in DMC or any other approach. Regarding computational aspects, calculation of each FN-DMC energies of Table II were performed using 800 cores on the Curie machine (TGCC/CEA/Genci) during about 15 h. The cost of deterministic CIPSI calculations to build the trial wavefunctions is marginal. Roughly speaking, the cost is similar to that needed for making CISD calculations with the same basis sets.

In Table III a selection of the best (lowest) values reported in the literature for the total energy of the water molecule is presented. Using DMC, the lowest value published so far is that of Clark et al. of -76.4368(4). Here, using the nodes of the CIPSI/cc-pCV5Z expansion an improved value of -76.437 44( 18) is obtained. The lowest upper bound reached using a post-Hartree Fock correlated approach is that of Almora-Dìaz of -76.4343, a value significantly higher than DMC values. Finally, the best (non-variational) estimates are those obtained by performing CBS extrapolation. At FCI level the most accurate one is that of Bytautas and Ruedenberg, 32 [START_REF] Klopper | [END_REF] -76.438 9 E 0 = -76.4390 (4). Here, our value of -76.438 94( 12) is, to the best of our knowledge, the most accurate value reported so far. In both cases the best experimentally derived estimate of -76.4389 is recovered within error bars.

In this study we have performed DMC calculations using nodes of multideterminant CI expansions obtained through a perturbative selection of the most important determinants (selected CI). In contrast with most QMC works, noreoptimization of nodes in presence of a Jastrow prefactor has been performed. For each basis set of the cc-pCVnZ family (n = 2-5), CIPSI nodes obtained are of near-Full-CI quality. As a result of the deterministic construction of nodes using CI expansions, the total fixed-node energy is found to be a smoothly decreasing function of the cardinal number of the basis set with a typical inverse third power law. The Complete Basis Set (CBS) limit leading to the total energy associated with exact nodes is then easy to perform. From a general perspective, we emphasize that employing selected CI nodes of increasing quality in a given family of basis sets may represent a simple, deterministic, reproducible, and systematic way of controlling the fixed-node error in DMC.

Communication:1

  Toward an improved control of the fixed-node error in quantum Monte Carlo: The case of the water molecule Michel Caffarel, 1 Thomas Applencourt, 1 Emmanuel Giner, 2 and Anthony Scemama 1 Laboratoire de Chimie et Physique Quantique, CNRS-Université de Toulouse, Toulouse, France 2 Dipartimento di Scienze Chimiche e Farmaceutiche, Universit degli Studi di Ferrara, Ferrara, Italy (Received 22 March 2016; accepted 6 April 2016; published online 20 April 2016)

  A.S. and M.C. thank the Agence Nationale pour la Recherche (ANR) for support through Grant No. ANR 2011 BS08 004 01. This work has been made through generous computational support from CALMIP (Toulouse) under the Allocation No. 2015-0510, and GENCI under the Allocation No. x2015081738.

TABLE I .

 I Number of determinants and corresponding variational energies for CIPSI expansions used in DMC for each cc-pCVnZ (n = 2 to 5) basis set. Last column: Deviations of the variational energy to the best FCI estimates of Almora-Dìaz.27 Energies in atomic units.

	Basis set	FCI size	# dets used in DMC	E var 0	FCI, Almora-Dìaz 27	Deviation
	cc-pCVDZ	∼10 10	172 256	-76.282 136	-76.282 865	0.0007
	cc-pCVTZ	∼2 • 10 14	640 426	-76.388 287	-76.390 158	0.0018
	cc-pCVQZ	∼2 • 10 17	666 927	-76.419 324	-76.421 148	0.0018
	cc-pCV5Z	∼7 • 10 19	1 423 377	-76.428 550	-76.431 105	0.0025

TABLE II .

 II All-electron DMC energies (in a.u.) obtained with CIPSI nodes for each basis set. Second column: Increase of CPU time due to the use of the large multideterminant expansion.FIG. 1. CBS extrapolation of FCI and DMC/CIPSI energies. Error bars on DMC data are plotted but almost imperceptible.

	Basis set[Ndets]	T CPU (Ndets)/T CPU (1det)	E DMC 0
	cc-pCVDZ[172 256]	∼101	-76.415 71(20)
	cc-pCVTZ[640 426]	∼185	-76.431 82(19)
	cc-pCVQZ[666 927]	∼128	-76.436 22(14)
	cc-pCV5Z[1 423 377]	∼235	-76.437 44(18)

TABLE III .

 III Comparison of nonrelativistic ground-state total energies of water obtained with the most accurate theoretical methods. Energies in a.u.

	Clark et al., 20 DMC (upper bound)	-76.436 8(4)
	This work, DMC (upper bound)	-76.437 44(18)
	Almora-Dìaz, 27 CISDTQQnSx (upper bound)	-76.434 3
	Helgaker et al., 29 R12-CCSD(T)	-76.439(2)
	Muller and Kutzelnigg, 30 R12-CCSD(T)	-76.437 3
	Almora-Dìaz, 27 FCI + CBS	-76.438 6(9)
	Halkier et al., 31 CCSD(T) + CBS	-76.438 6
	Bytautas and Ruedenberg, 32 FCI + CBS	-76.439 0(4)
	This work, DMC + CBS	-76.438 94(12)
	Experimentally derived estimate