Kinetic theory for the coherent scattering function S(q) of classical liquids

Within the formalism of Mori and Zwanzig a kinetic equation for the phase-space density of simple classical liquids is derived whose relaxation kernel is obtained as the solution of a second kinetic equation. The latter equation is closed by introducing a phenomenological relaxation rate. The solution of the resulting equations can be reduced to the evaluation of the self-correlation function for a tagged particle, studied earlier by the present authors. The calculated scattering functions S(q) for liquid argon and rubidium are compared with neutron scattering experiments as well as molecular dynamics calculations. For liquid argon the earlier results of Duderstadt and Akcasu are improved, and it is furthermore shown that the present theory adequately accounts for the persistence of propagating density fluctuations in liquid rubidium.