The velocity autocorrelation function and the binary diffusion coefficient of a single hard sphere test particle whose mass and size was less than that of the solvent were computed in the dense fluid phase. Large deviations from the Enskog theory of uncorrelated motion were found as the diffusion coefficient vanishes for the lighter and larger test particle at higher densities. Comparison with experimental values led to the conclusion that the correlated motions observed in hard spheres correspond well to those observed in real systems.

Studies in Molecular Dynamics XIV. Mass and Size Dependence of the Binary Diffusion Coefficient. Alder, B.J., Alley, W.E., Dymond, J.H.: The Journal of Chemical Physics, 1974, 61[4], 1415-20