Self-diffusion of particles interacting through a square-well or square-shoulder potential

The diffusion coefficient and velocity autocorrelation function for a fluid of particles interacting through a square-well or square-shoulder potential are calculated from a kinetic theory similar to the Davis-Rice-Sengers theory and the results are compared to those of computer simulations. At low densities the theory yields too low estimates due to the neglect of correlations between subsequent partial collisions of identical pairs; in particular, the neglect of boundstate effects appears important. At intermediate densities the theory makes reasonable predictions and at high densities it produces too high values, due to the neglect of ring terms and other correlated collision events. The results for the square-shoulder potential generally exhibit better agreement between theory and simulations than do those for the square-well potential