Density dependence of sound dispersion in repulsive Lennard-Jones fluids

We performed molecular dynamics simulations on a system of 864 particles interacting through a repulsive Lennard-Jones potential at a reduced temperature $k_B$T/ɛ=0.97 and at 11 different densities to determine the sound dispersion as a function of wave number and density. As the density increases a propagation gap appears in the sound-dispersion curve, as predicted by the revised Enskog theory for hard spheres. The gap disappears again at densities well before the undercooled fluid phase, similar as in the Lennard-Jones systems and in liquid argon.