Anisotropy of the Proton Momentum Distribution in Water

One
of the many peculiar properties of water is the pronounced
deviation of the proton momentum distribution from Maxwell–Boltzmann
behavior. This deviation from the classical limit is a manifestation
of the quantum mechanical nature of protons. Its extent, which can
be probed directly by deep inelastic neutron scattering experiments,
gives important insight on the potential of mean force felt by H atoms.
The determination of the full distribution of particle momenta, however,
is a real tour de force for both experiments and theory, which has
led to unresolved discrepancies between the two. In this study, we
present comprehensive, fully converged momentum distributions for
water at several thermodynamic state points, focusing on the components
that cannot be described in terms of a scalar contribution to the
quantum kinetic energy, and providing a benchmark that can serve as
a reference for future simulations and experiments. In doing so, we
also introduce a number of technical developments that simplify and
accelerate greatly the calculation of momentum distributions by means
of atomistic simulations.