The second-order and fourth-order frequency sum rules of the velocity auto-correlation function were evaluated, for 6 thermodynamic states along the liquid-vapor coexistence curve, by using the Ashcroft pseudopotential and a corresponding pair distribution function that had been obtained by means of molecular dynamics simulations. These sum rules, and a model for the self-diffusion coefficient, were used to study the time evolution of the velocity auto-correlation function and the self-diffusion coefficients. The results were compared with molecular dynamics simulation data. It was found that the present model provided the first semi-quantitative explanation for the density and temperature dependences of the velocity auto-correlation function and the self-diffusion coefficients of expanded Rb.

S.K.Sharma, K.Tankeshwar: Journal of Physics - Condensed Matter, 1996, 8[50], 10839-45