Nuclear quantum effects in water

In this work, a path integral Car-Parrinello molecular dynamics\footnote{CPMD V3.11 Copyright IBM Corp 1990-2006, Copyright MPI fuer Festkoerperforschung Stuttgart 1997-2001.} simulation of liquid water is performed. It is found that the inclusion of nuclear quantum effects systematically improves the agreement of first-principles simulations of liquid water with experiment. In addition, the proton momentum distribution is computed utilizing a recently developed ``open'' path integral molecular dynamics methodology\footnote{J.A. Morrone, V. Srinivasan, D. Sebastiani, R. Car \emph{J. Chem. Phys.} \textbf{126} 234504 (2007).}. It is shown that these results, which are consistent with our computations of the liquid structure, are in good agreement with neutron Compton scattering data\footnote{G.F. Reiter, J.C. Li, J. Mayers, T. Abdul-Redah, P. Platzman \emph{Braz. J. Phys.} \textbf{34} 142 (2004).}. The remaining discrepancies between experiment and the present results are indicative of some degree of over-binding in the hydrogen bond network, likely engendered by the use of semi-local approximations to density functional theory in order to describe the electronic structure.