DENSITY EXPANSION OF THE DYNAMIC STRUCTURE FACTOR FROM MOLECULAR-DYNAMICS SIMULATION

The dynamic structure facto S(k,omega) of low-density Ar-36 measured in a recent small-angle inelastic neutron scattering experiment has been analysed for the first time in terms of a virial (density) expansion. and the first (i.e. linear) density correction to the free-gas S(k,omega) has been compared with the prediction of Enskog kinetic theory for hard spheres. It is demonstrated that the dynamics of density fluctuations is a sensitive probe of the interatomic pair potential. and in order to have a comparison of the experimental results with calculations based on a more realistic potential. we have performed molecular dynamics simulations of a Lennard-Jones system, from which the corresponding time correlation function F(k,t) has been obtained at various densities. The computations are very time-consuming for the following reasons. (i) A large box size is needed to have a range of wavevectors as small as those required for the effect to be detectable. In fact, deviations from free-gas (i.e. single molecule dynamics tend to vanish at k almost-equal-to 1.5 nm-1. (ii) The free-ps contribution to F(k,t), which must be subtracted in order to extract the density dependence, accounts for about 90% of the total correlation function, so that a very accurate simulation is required, (iii) The statistical uncertainties must be small enough for a reasonably precise extraction of the linear density dependence and for the determination of the (k,t)-dependent density range beyond which higher-order deviations begin to be visible. Some preliminary results are presented here where the linear density coefficient is determined and compared with both the experimental results and the hard-sphere theory.