Effects of correlated collisions on atomic diffusion in a hard-sphere fluid

Abstract

The velocity autocorrelation function in a hard-sphere fluid is calculated by including corrections to the Boltzmann-Enskog theory due to correlated binary (ring) collisions. The memory function describing these processes is reduced to a form suitable for numerical computation by assuming the dominant contributions are due to couplings between hydrodynamic modes. A simple approximation is introduced to account for the effects of nonhydrodynamic modes. Using the interpolation formulas recently suggested by Resibois, we have computed the memory function, the velocity autocorrelation function, and the self-diffusion coefficient $D$. The diffusion coefficient shows the enhancement effect relative to the Enskog diffusion coefficient that was first observed by computer molecular-dynamics calculations. Our results also show a rapid decrease in $D$ in the high-density region. A comparison of the present work with the analysis of Resibois is made, and further refinements are suggested.