Molecular-dynamics study of the dependence of self-diffusion on system size in a dense binary liquid of hard spheres

The dependence on system size of self-diffusion in a dense binary liquid of hard spheres was studied by molecular dynamics simulation of two systems with 216 and 5832 particles. The ratio of particle radii is alpha = 0.85. The relative difference between the self-diffusion coefficients D-s for both system sizes increases from 11% to 18% as the packing fraction phi is increased from 0.533 to 0.563. The relation between the size dependence of D-s and the underlying size dependence of the velocity-autocorrelation function (VAF) was studied in detail for phi = 0.548. For intermediate and long times the difference Delta psi (t) of the VAFs for the two sizes is well reproduced by the mode-coupling formula expressing the coupling of self-diffusion to modes of collective flow. However, the short-time part of Delta psi (t) is equally important. A simpler relation was found between the size dependence of D-s and that of the memory function M(t) associated with the VAF. M(t) is larger and decays more slowly for the smaller systems, which leads to the observed size dependence of D-s. (C) 2001 Elsevier Science B.V. All rights reserved.