Translational molecular motion and cages in computer molecular liquids

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.