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Quantum Monte Carlo methods are accurate and promising many body techniquesfor electronic structure calculations which, in the last years, areencountering a growing interest thanks to their favorable scaling with thesystem size and their efficient parallelization, particularly suited for themodern high performance computing facilities. The ansatz of the wave functionand its variational flexibility are crucial points for both the accuratedescription of molecular properties and the capabilities of the method totackle large systems. In this paper, we extensively analyze, using differentvariational ansatzes, several properties of the water molecule, namely: thetotal energy, the dipole and quadrupole momenta, the ionization and atomizationenergies, the equilibrium configuration, and the harmonic and fundamentalfrequencies of vibration. The investigation mainly focuses on variational MonteCarlo calculations, although several lattice regularized diffusion Monte Carlocalculations are also reported. Through a systematic study, we provide a usefulguide to the choice of the wave function, the pseudo potential, and the basisset for QMC calculations. We also introduce a new strategy for the definitionof the atomic orbitals involved in the Jastrow - Antisymmetrised Geminal powerwave function, in order to drastically reduce the number of variationalparameters. This scheme significantly improves the efficiency of QMC energyminimization in case of large basis sets.

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