Analysis of the role of attractive forces in self-diffusion of a simple fluid

The role of the attractive force in self-diffusion of atoms in a Lennard-Jones fluid is explored through molecular dynamics simulation. Expressions for the self-diffusion coefficient in terms of the memory function and the random force autocorrelation function are used to develop simple approximations which separate the contribution of attractive and repulsive forces to self-diffusion. A result of the analysis is the definition of an effective hard-sphere radius for dynamic properties. The expression describing the effect of the attractive interactions on the rate of self-diffusion in a fluid at low density is found to predict successfully the dependence at much higher densities and k B T ⩾ ϵ.