Velocity-autocorrelation spectrum of simple classical liquids

The velocity-autocorrelation function of a tagged particle moving in a classical liquid is expressed in terms of a characteristic oscillator frequency and a frequency-dependent relaxation kernel. The relaxation spectrum is approximated by calculating the interaction of the tagged particle with current excitations of the liquid. The interaction with the longitudinal modes is shown to be responsible for the observed peak structure of the correlation function. The results of the present theory, in particular the values for the diffusion constant, agree well with the molecular-dynamics experiments on argon and rubidium.