  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.

Chemical Physics;Strongly Correlated Electrons;Atomic Physics;Computational Physics;Quantum Physics