MOLECULAR-DYNAMICS SIMULATION ON SIMPLE FLUIDS - DEPARTURE FROM LINEARIZED HYDRODYNAMIC BEHAVIOR OF THE DYNAMICAL STRUCTURE FACTOR

Large scale molecular-dynamics simulations have been carried out that are especially designed to explore the intermediate region of the (k, w) plane of simple fluids, in order to bridge the gap between the hydr odynamics and kinetic domains; the gap is, unfortunately, unreachable by present light and neutron scattering techniques. The density fluctu ations were obtained in a wide range of wave vectors and frequencies, i.e., 0.007 < [k (angstrom-1)] < 0.4 and 0.006 < [omega (THz)] < 100, respectively, for a state point of Ar-36 close to solidification. The dynamical structure factors are analyzed under linearized hydrodynamic s assumptions, and the departure from such behavior is studied through sound damping factor and the Landau-Placzec ratio for k > 0.15 angstr om-1. The deviations of the thermal damping factors appear at k > 0.05 angstrom-1, which can be described in terms of the Fixman correction with a correlation length of congruent-to 15 angstrom. The dispersion curve of the sound peak gives an adiabatic sound velocity slightly sma ller than the one obtained from thermodynamic derivatives, in qualitat ive agreement with light scattering data.