Structural and dynamical behavior of model fluids at high and low densities

Abstract

The Lennard-Jones (LJ) and the ‘hard soft sphere’ (HSS) system are studied at high and very low density by molecular dynamics (MD) calculations. Comparsion of the various pair correlation functions shows the governing role of attractive forces at low densities and the dominating influence of repulsive forces at liquidlike densities. The results at low densities show clearly the ‘long-ranged’ structure of the LJ potential, which cannot be noticed at high densities. The well-known fact that the viscosity is proportional to the density at high density states and inversely proportional at low density is quantitatively studied. In particular, the relaxation times of the momentum and positional configurations are investigated by the momentum and force autocorrelation function as well as scattering functions. The results for the relaxation processes analyzed in terms of the momentum and force correlation functions differ completely from those studied by the collective scattering functions. Only the collective processes exhibit the density-dependent behavior assumed recently to explain the inversion of the density dependence of the viscosity constant by kinetic theorectical arguments.