Molecular dynamics simulations are used to determine the time-dependent friction for pair diffusion in an isotropic Lennard-Jones fluid as a function of the separation between two diffusing particles. A numerical method proposed by Straub, Borkovec and Berne is used. It is found that both the initial value and the detailed time-dependence of the friction are dependent on the interparticle separation. The dependence of the pair diffusion coefficient on separation is determined. Comparisons are made with various hydrodynamic and collision theories. The rate constant for diffusion controlled reactions is discussed.
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Straub et al. (1990) studied this question.