Molecular dynamics simulation of the structure of liquid rubidium along the saturated vapour-pressure curve

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Abstract
The structure and the thermal motion of ions in liquid rubidium are simulated for six states including the critical point along the saturated vapour-pressure curve using the molecular dynamics method. The effective pair potential between rubidium ions in expanded liquid states is calculated by extrapolating the optimised model potential parameters proposed by Cowley (1976) and the screening parameters of Singwi et al. (1970). It becomes deeper and softer at lower densities. The calculated structure factor, S(Q) and the pair distribution function, g(r) are found in better agreement with the measured ones than those calculated by Mountain (1978) using the empty-core pair potential of Price (1971). The calculated short-range orders for an expanded state close to the critical point are significantly different from those of liquid argon near the critical point, which is mostly due to softening of repulsion between rubidium ions. The self-diffusion constant is calculated and fits Tn law with n=1.65 along most of the saturated vapour-pressure curve, in contrast to the case of fluid argon where it fits the Arrhenius-type formula.