High-Frequency Expansion of Memory Function in Classical Simple Liquids

Abstract

Transverse collective motion in classical simple liquids is studied by using a high-fre· quency expansion of memory functions for the transverse current correlation function. Ex­ plicit expressions for the 'first, second, fourth and sixth frequency moments of the spectrum of the correlation function are obtained, in which atomic correlations in the liquids are taken into account up to quartets. As an application, the dispersion curve of transverse collective modes in liquid argon is calculated numerically. As in the case of longitudinal collective modes, fairly good agreement with the result obtained by Rahman using molecular dynamics calculations is obtained, provided that an approximate cancellation is assumed to take place between a term containing a three-body correlation function and those with two-body corre­ lation functions which appear in an expansion term giving the fourth frequency moment of the spectrum. By applying Kirkwood's superposition approximation, discussion is given on the role of the three-body correlation function in determining the dispersion of transverse collective modes.