Interparticle velocity correlations in simple liquids

The momentum transfer of a test particle to a cluster of atoms initially in the first shell of neighbors is investigated at liquid densities by molecular dynamics methods. The comparison of the results obtained for Lennard-Jones and soft spheres models is discussed. In the short time regime the interpretation of the data is shown to parallel that of the single particle velocity autocorrelation function. At longer times, a simple physical model can account for the decay of the cross correlation in terms of the increasing number of atoms involved in the process.