Translational Density of States Derived from Short-Time Relative Motions

Abstract

The translational dynamics of tagged pairs of molecules that are in the first coordination shell with specific relative orientations at the initial time is studied in the short-time domain. The partition of the pair angular space in orientational classes of dimer geometries allows one to detect normal modes along particular spatial directions and to select a part of the translational density of states. Applied to liquid water at 245 K, the technique yields evidence of the existence of local modes at about 60 and $230{\mathrm{cm}}^{-1\mathrm{}}$, and provides a tool for explaining their microscopic origin.