Molecular dynamics study of the velocity cross-correlations in liquids

Abstract

Velocity cross-correlations for both soft-sphere fluids at different densities and temperatures and a simple molten salt are investigated by molecular dynamics simulation. Time correlation functions between the velocity of a tagged particle and velocities of particles within specified ranges of initial separations are calculated. In the case of the soft-sphere fluids, separation ranges corresponding to the first, second and third shells of neighbors are considered whereas up until six different shells are analyzed for the molten salt. The calculated functions allow us to build a picture of the spread of the initial momentum of a tagged particle over the successive shells of neighbors. It is observed that collisions with intermediate particles are the main mechanism for the transfer of momentum. A balance between the momentum exchanged by particles in a given shell with those in the two adjacent shells should be carried out in order to analyze the resulting velocity cross-correlation functions. The rate of transfer of momentum between distant particles increases with the density and temperature of the liquid. It has been noticed an incipient coherence, which is more marked for the ionic melt, between the motions of atoms in nonadjacent shells.