Long-time behavior of the velocity autocorrelation function for moderately dense, soft-repulsive, and Lennard-Jones fluids.

The long-time behavior of the velocity autocorrelation function (VAF), for hard disk and sphere systems, has been extensively explored. Its behavior for systems interacting via a soft repulsive or attractive potential is less well known. We explore the conditions under which the nonexponential, long-time tail in the velocity autocorrelation function of a tagged atom, in soft-repulsive sphere (Weeks-Chandler-Andersen) and Lennard-Jones atomic fluids, may be readily observed by the molecular dynamics method. The effect of changing the system size, the fluid density, the form of the interatomic force and the mass of the tagged atoms are investigated. We were able to observe this long-time tail only for systems of moderate density. At low density the effect, if it exists, is at longer times than we can currently simulate owing to limitations of system size and at higher densities these tails were not observed possibly due to other effects dominating the behavior of the VAF and masking this behavior. Under the physical conditions that are simulated here attractive forces have very little effect on the behavior of the VAF. However, as the mass of the tagged particles is increased the time at which the long-time tail commences is lengthened and its magnitude is significantly increased. This later effect suggests that by increasing the mass of the tagged particles one may be able to study more readily the behavior, nature and physical origin of long-time behavior of the VAF both by computational and by experimental techniques.