Translational molecular motion and cages in computer molecular liquids

Abstract

The probability density for changes in direction of molecular motion was investigated as a function of time in simulations of CF₄ and CS₂ near their triple points. The probability of forward and reversed motion shows features related to the shape of the velocity correlation function. These features are enhanced in a group of molecules chosen with high initial kinetic energies. This and other observations lead to the suggestion that the motion of high energy molecules is primarily a rattling within the cage of nearest neighbours, while low energy molecules move in a diffusional way. The life time of the cage defined by the nearest neighbours is long compared to the velocity correlation times.