Binary-collision contributions to atomic motions in fluids

Abstract

Binary-collision contributions to the first order space-time memory function in the Mori-Zwanzig representation of a self-current autocorrelation function have been obtained. The expression for the memory function involves the static pair correlation function and the time dependence of the position and the momentum of a particle moving in a central potential. In the limit of long wavelength, the memory function reduces to that for the velocity autocorrelation function (VACF). The self-diffusion coefficient has been found to reduce to the Enskog value in the limit of a hard sphere interaction. Numerical results for the VACF, its memory function, and the self-diffusion coefficient for a Lennard-Jones fluid have been obtained for several thermodynamic states. These results are compared with available molecular dynamics data to identify the extent to which the binary-collision approximation is valid.