Molecular properties by Quantum Monte Carlo: an investigation on the role of the wave function ansatz and the basis set in the water molecule

TL;DR

This study evaluates how different wave function ansatzes and basis sets affect the accuracy of Quantum Monte Carlo calculations for water molecule properties, providing practical guidance and a new orbital definition strategy.

Contribution

It introduces a novel orbital definition method that reduces variational parameters and enhances efficiency in QMC energy minimization for large basis sets.

Findings

Wave function ansatz choice significantly impacts property accuracy.

The new orbital strategy improves energy minimization efficiency.

Guidelines for selecting wave functions, pseudopotentials, and basis sets are provided.

Abstract

Quantum Monte Carlo methods are accurate and promising many body techniques for electronic structure calculations which, in the last years, are encountering a growing interest thanks to their favorable scaling with the system size and their efficient parallelization, particularly suited for the modern high performance computing facilities. The ansatz of the wave function and its variational flexibility are crucial points for both the accurate description of molecular properties and the capabilities of the method to tackle large systems. In this paper, we extensively analyze, using different variational ansatzes, several properties of the water molecule, namely: the total energy, the dipole and quadrupole momenta, the ionization and atomization energies, the equilibrium configuration, and the harmonic and fundamental frequencies of vibration. The investigation mainly focuses on variational Monte Carlo calculations, although several lattice regularized diffusion Monte Carlo calculations are also reported. Through a systematic study, we provide a useful guide to the choice of the wave function, the pseudo potential, and the basis set for QMC calculations. We also introduce a new strategy for the definition of the atomic orbitals involved in the Jastrow - Antisymmetrised Geminal power wave function, in order to drastically reduce the number of variational parameters. This scheme significantly improves the efficiency of QMC energy minimization in case of large basis sets.