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A study of the velocity autocorrelation function for hard spheres over the entire fluid density region shows that deviations from exponential behavior are small. At very low density a small positive deviation is found which is accounted for in the Boltzmann theory of the self-diffusion coefficient. At higher densities the principal non-Markovian process is identified with a surprisingly long persistence of velocity currents. At still higher densities, near solidification, anticorrelating backscattering events predominate.
Alder et al. (Mon,) studied this question.