Microscopic collective dynamics in liquid para-H2

Well-defined phonons with strong anomalous (upward) dispersion are observed by inelastic neutron scattering in liquid $para\ensuremath{-}{\mathrm{H}}_{2}$ at a temperature of 15.7 K. The damping, being very small for the low-$Q$ phonons, increases with wave vector Q, and only broad features are observed for $Q\ensuremath{\gtrsim}1{\mathrm{\AA{}}\mathrm{}}^{\ensuremath{-}1}.$ This behavior is shown to deviate strongly from results of molecular simulations of a fully classical analogue using a realistic potential.