MOLECULAR-DYNAMICS STUDY OF THE VELOCITY CROSS-CORRELATIONS IN LIQUIDS

Velocity cross-correlations for both soft-sphere fluids at different d ensities 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 spec ified ranges of initial separations are calculated. In the case of the soft-sphere fluids, separation ranges corresponding to the first, sec ond and third shells of neighbors are considered whereas up until six different shells are analyzed for the molten salt. The calculated func tions allow us to build a picture of the spread of the initial momentu m of a lagged particle over the successive shells of neighbors. It is observed that collisions with intermediate particles are the main mech anism for the transfer of momentum. A balance between the momentum exc hanged by particles in a given shell with those in the two adjacent sh ells 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 li quid. It has been noticed an incipient coherence, which is more marked for the ionic melt, between the motions of atoms in nonadjacent shell s, (C) 1998 American Institute of Physics. [S0021-9606(98)51025-2].