Velocity cross-correlations and atomic momentum transfer in simple liquidswith different potential cores

Time correlation functions between the velocity of a tagged particle and ve locities of particles within specified ranges of initial separations have b een obtained by molecular dynamics simulation These correlation functions h ave allowed us to analyze the momentum transfer between particles in differ ent coordination shells. Two simple liquids at very different densities and two purely repulsive potentials with very different softnesses have been c onsidered. The longitudinal correlations, which are the velocity cross-corr elations along the initial direction defined by the centers of two given pa rticles, have been calculated separately. I has been proven that these corr elations should he attributed to particles both in front of and behind the central one. As with propagating longitudinal modes, they are strongly depe ndent on the softness of the potential core. Some characteristic features o f the velocity correlation functions after the initial rise should be relat ed to nonlongitudinal correlations. It has been shown that velocity cross-c orrelations between distinct particles cannot only be attributed to the dir ect interactions among particles, but also to the motions induced by the mo vement of a tagged particle on their neighbors.