FisMat2017 - Submission - View

Abstract's title: Fully Quantum Description of Water Clusters: Combining Variational Quantum Monte Carlo with Path Integral Langevin Dynamics
Submitting author: Michele Casula
Affiliation: Université Pierre et Marie Curie
Affiliation Address: Institut de Minéralogie, de Physique des Matériaux et de Cosmochimie, Université Pierre et Marie Curie, 4 place Jussieu
Country: France
Oral presentation/Poster (Author's request): Oral presentation
Other authors and affiliations: Félix Mouhat (IMPMC, Paris, France), Sandro Sorella (SISSA, Trieste, Italy), Rodolphe Vuilleumier (ENS, Paris, France), Antonino Marco Saitta (IMPMC, Paris, France)

We introduce a novel approach for a fully quantum description of coupled electron–ion systems from first principles. It combines the variational quantum Monte Carlo (QMC) solution of the electronic part with the path integral formalism for the quantum nuclear dynamics. Nuclear quantum fluctuations are included via a set of fictitious classical particles (beads), linked by harmonic interactions, whose dynamics is driven by accurate ionic forces computed at the QMC level. The stochastic noise affecting the QMC forces contributes to thermalize the particles in a path integral Langevin dynamics framework. Our general algorithm relies on a Trotter breakup between the dynamics driven by ionic forces and the one set by the harmonic interbead couplings. The latter is exactly integrated, even in the presence of the Langevin thermostat, thanks to the mapping onto an Ornstein–Uhlenbeck process. This framework turns out to be also very efficient in the case of noiseless (deterministic) ionic forces. The new implementation is first validated on the Zundel ion (H5O2+) by direct comparison with standard path integral Langevin dynamics calculations made with a coupled cluster potential energy surface. We then applied our method to the protonated water hexamer, where we computed thermal distribution functions, provided for the first time by a method which explicitely includes electron correlation, together with thermal and quantum nuclear effects.