BINARY-COLLISION CONTRIBUTIONS TO ATOMIC MOTIONS IN FLUIDS

Binary-collision contributions to the first order space-time memory fu nction in the Mori-Zwanzig representation of a self-current autocorrel ation function have been obtained. The expression for the memory funct ion involves the static pair correlation function and the time depende nce of the position and the momentum of a particle moving in a central potential. In the limit of long wavelength, the memory function reduc es 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 Lennar d-Jones fluid have been obtained for several thermodynamic states. The se results are compared with available molecular dynamics data to iden tify the extent to which the binary-collision approximation is valid.