Quantum Effects on Liquid Dynamics as Evidenced by the Presence of Well-Defined Collective Excitations in Liquidpara-Hydrogen

The origin of the well-defined collective excitations found in liquid $\mathrm{para}-{\mathrm{H}}_2$ by recent experiments is investigated. The persistence of their relatively long lifetimes down to microscopic scales is well accounted for by calculations carried out by means of path-integral-centroid molecular dynamics. In contrast only overdamped excitations are found in calculations carried within the classical limit. The results provide fully quantitative evidence of quantum effects on the dynamics of a simple liquid.