Comment on "collective modes in fluids and neutron scattering"

de Schepper and Cohen have shown recently that the hydrodynamic modes for a hardsphere fluid may be extended to quite large wave vectors, and that at high densities the hydrodynamic part of the dynamical-structure factors dominates over a wide range of wave vectors. The objective here is to describe the results of a similar analysis of two simple models of the self-structure factor. The simplicity of the models allows a scaling of the frequency and wave vectors such that a universal form independent of potential model, density, and temperature is obtained. The diffusion mode and hydrodynamic part of the self-structure factor are calculated, leading to a behavior similar to that observed by de Schepper and Cohen.