Hard-sphere kinetic-theory analysis of classical, simple liquids

The Enskog transport equation, suitably modified to give the proper short-time behavior, is used to study the phase-space density correlation function of a dense fluid of hard spheres. The method of kinetic models is used to obtain numerical solutions for the dynamic structure factor $S(Q, \ensuremath{\omega})$; in particular, results are obtained for liquid argon. It is found that for the hard-sphere calculations to give a satisfactory description of the available experimental data produced by computer molecular dynamics studies and neutron inelastic scattering measurements, the factor $g({r}_{0})$, the pair distribution at contact, should be replaced by a wavelength-dependent quantity. Moreover, the wavelength dependence determined by fitting the experimental data shows a close correlation with the well-known behavior of the static structure factor $S(Q)$. Possible reasons for a nonlocal $g({r}_{0})$ are discussed.