Two-body contribution to the density fluctuations in a dilute gas from molecular dynamics simulation

We have performed molecular-dynamics simulations of the intermediate scattering function F(k,t) of a Lennard-Jones gas at room temperature, low densities, and very low k values in order to study the density dependence of the deviations from the free-gas dynamics. We have extracted the linear term in the density expansion of these deviations as a function of time, which provides information on the two-body interaction potential. We compare these results with those obtained recently from a neutron inelastic scattering experiment on 36Ar, performed in the same range of thermodynamical states and with a theoretical calculation for hard spheres. The comparison confirms the experimental results and shows unambiguously that the linear term in a density expansion of F(k,t) and of the dynamic structure factor S(k,?) is a sensitive probe of the details of the pair interaction.