Exact calculation of the linear term in the density expansion of the dynamic structure factor of a dilute gas.

We evaluate the linear term in the density expansion of the dynamic structure factor for a classical gas using a generalized Enskog theory developed by one of the authors, which enables us to describe the dynamics at small time and length scales where the use of the Boltzmann equation is severely limited. Agreement of the theory with experiment and simulation is very good. We find that the linear term is very sensitive to the shape of the potential so that the dynamic structure factor can serve as a good probe to determine the intermolecular potentials of dilute gases.