Molecular-dynamics study of dynamical properties of dense soft-sphere fluids: The role of short-range repulsion of the intermolecular potential

Isokinetic molecular-dynamics simulations have been performed for soft-sphere fluids of 4th-, 6th-, 9th-, and 12th-inverse-power potentials near the respective freezing point. We have investigated how various dynamical properties of these fluids are influenced by the softness of the potentials to which some groups of material are attributed. The reduced diffusion constant is found to be insensitive to the choice of the softness of the potential. On the other hand, the frequency-dependent self-motion, as characterized by the spectrum of the velocity autocorrelation function, shows a pronounced dependence on the softness of the potential. The collective dynamics has also been studied in terms of the dynamic structure factor, which has been calculated over a wide range of wave numbers, covering the wave number corresponding to the first maximum of the static structure factor. A persistent sound wave is observed for the softer potentials such as 4th- and 6th-inverse-power potentials, which is in remarkable contrast to the results obtained for the steeper potentials such as 9th- and 12th-inverse-power potentials. These significant dynamical properties, dependent on the softness parameter, are consistent with the dynamical behavior observed in liquid alkali metals and liquified inert gases.