Dynamic properties of simple liquids: Dependence on the softness of the potential core

The dependence of both individual and collective dynamic properties of simp le liquids on the steepness of the potential cores is analyzed. Molecular d ynamics simulations of liquids in identical conditions but assuming two rep ulsive interaction potentials with very different softness have been carrie d out. Two liquids at very different densities (with volume packing fractio ns eta=0.5 and eta=0.3) have been considered for each potential. The study of the dynamic collective properties includes the dynamic structure factor and the longitudinal and transverse current correlations. It has been corro borated that longitudinal modes associated with density fluctuations propag ate up to higher wave numbers in liquids with softer potential cores. Howev er, the propagating transversal modes in dense liquids are weakly influence d by the hardness of the potential core. Other properties such as the sound velocity in liquids at high density or the dynamic structure factor at the kinetic regime are rather insensitive to the details of the repulsive inte ractions. The velocity correlation functions for liquids with soft repulsiv e potential walls are markedly oscillatory. These oscillations are associat ed with the coupling of the atomic velocity to collective density fluctuati ons. (C) 1999 American Institute of Physics. [S0021-9606(99)51046-5].