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 was α = 0.85. The relative difference between the self-diffusion coefficients Ds for both system sizes increased from 11% to 18% as the packing fraction φ was increased from 0.533 to 0.563. The relation between the size dependence of Ds and the underlying size dependence of the velocity-autocorrelation function was studied in detail for φ = 0.548. For intermediate and long times the difference Δψ(t) of the VAFs for the two sizes was well reproduced by the mode-coupling formula expressing the coupling of self-diffusion to modes of collective flow. However, the short-time part of Δψ(t) was equally important. A simpler relation was found between the size dependence of Ds and that of the memory function M(t) associated with the velocity-autocorrelation function. M(t) was larger and decays more slowly for the smaller systems, which led to the observed size dependence of Ds.

Molecular-Dynamics Study of the Dependence of Self-Diffusion on System Size in a Dense Binary Liquid of Hard Spheres. Jäckle, J., Kawai, H.: Physica A, 2001, 291[1-4], 184-96