Frequency-Dependent Friction Constant Analysis of Diffusion in Simple Liquids

A simple discussion of the self-diffusion motions of an atom in a simple liquid is given. The discussion, based mainly on fluctuation-dissipation theorems, dispersion relations, and sum rules, provides a rigorous framework in which a number of phenomenological descriptions are examined. Specific results for liquid argon are compared with computer molecular dynamics calculations. It is shown that the use of simple frequency-dependent friction constants leads to reasonable calculations of the velocity autocorrelation function and of the self-diffusion coefficient.