SELF-DIFFUSION COEFFICIENTS OF EXPANDED RUBIDIUM

The second-order and fourth-order frequency sum rules of the velocity autocorrelation function (VACF) of Rb have been evaluated for six ther modynamic states along the liquid-vapour coexistence curve by using th e Ashcroft pseudopotential and corresponding pair distribution functio n obtained by molecular dynamics (MD) simulation. These sum rules and a model for the self-diffusion coefficient have been used to study the time evolution of the VACF and self-diffusion coefficients. The resul ts obtained have been compared with MD simulation data. It is found th at our model provides the first semiquantitative explanation for the d ensity and temperature dependences of the VACF and self-diffusion coef ficients of expanded Rb.