  A microscopic theory of molecular motion in classical monatomic liquids,
proposed by Glass and Rice [Phy. Rev. 176, 239 (1968)], is revisited and
extended to incorporate the dynamic friction in the Brownian description of the
atomic diffusion in a mean-time-dependent harmonic force field. A modified,
non-Markovian Langevin equation is utilized to derive an equation of motion for
the velocity autocorrelation function with time-dependent friction coefficient.
Numerical solution of the equation gives an excellent account of the velocity
autocorrelation function in LJ liquids, liquid alkali and transition metals
over a broad range of density and temperature. Derivation of the equation of
motion leads to a self-consistent expression for the time-dependence of
friction coefficient. Our results demonstrate that the nature of
time-dependence of the friction coefficient changes dramatically with the
liquid density. At low and moderate densities, the dynamic friction decays
exponentially whereas it increases exponentially at high liquid densities. Our
findings provide an opportunity to have a new outlook of the Brownian
description of atomic dynamics in liquids.
