title: Communication: Toward an improved control of the fixed-node error in quantum Monte Carlo: The case of the water molecule.
abstract: All-electron Fixed-node DiffusionMonte 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 numbern 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 interactionnodes 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 diffusionMonte Carlo.
authors.affiliation: Laboratoire de Chimie et Physique Quantique, CNRS-Université de Toulouse, Toulouse, France.
authors.forename: Michel
authors.initials: M
authors.lastname: Caffarel
authors.affiliation: Laboratoire de Chimie et Physique Quantique, CNRS-Université de Toulouse, Toulouse, France.
authors.forename: Thomas
authors.initials: T
authors.lastname: Applencourt
authors.affiliation: Dipartimento di Scienze Chimiche e Farmaceutiche, Universit degli Studi di Ferrara, Ferrara, Italy.
authors.forename: Emmanuel
authors.initials: E
authors.lastname: Giner
authors.affiliation: Laboratoire de Chimie et Physique Quantique, CNRS-Université de Toulouse, Toulouse, France.
authors.forename: Anthony
authors.initials: A
authors.lastname: Scemama