Nuclear quantum effects in water: A study in position and momentum space

Water is a system of fundamental importance in the biological and physical sciences. Due to the low mass of the proton, nuclear quantum effects are non-negligible in water. These effects may be included in atomic simulation via the Feynman path integral methodology, and have been studied over the past two decades.;Recently, neutron Compton scattering experiments have been performed that have uncovered the proton momentum distribution in a variety of systems, including water. This momentum distribution significantly varies from the classical (Boltzmann) result, and is sensitive to the potential energy surface. We have developed an "open" path integral molecular dynamics methodology in order to compute the proton momentum distribution in water. We have performed the simulation utilizing both empirical force fields and the Car-Parrinello methodology, which derives the interactions "on the fly" from first principles electronic structure theory.;We find that the computed momentum distributions, although in fairly good agreement with experiment, are somewhat broader than the scattering results. This indicates, via the uncertainty principle, that the proton is somewhat more localized in the simulation than in experiment. We also find that the Car-Parrinello result, unlike the force field outcome, is capable of qualitatively reproducing the experimental differences between the liquid water and ice distributions. These differences arise from the red-shift in the OH stretch that occurs when transitioning from liquid to solid water. Such effects are a reflection of the entanglement of the potential energy surface with the momentum distribution that is engendered by the uncertainty relation between position and momentum.;In addition, we find that nuclear quantum effects broaden the structure of liquid water as compared to Car-Parrinello results with classical nuclei. Accordingly, there is an increased fraction of broken hydrogen bonds that is found in the path integral result. Although this effect is well-known in force field studies, this is a new finding in first principles water.;We have also carried out simulations of high-pressure phases of ice in order to study proton tunneling and delocalization in light of recent experimental proton momentum distributions. Preliminary results on this topic are presented in this work. An attempt to develop an algorithm which improves the computational efficiency of path integral simulations in aqueous systems is also presented.