LONG-TIME TAILS IN 2-DIMENSIONAL FLUIDS BY MOLECULAR-DYNAMICS

We report on molecular dynamics simulation of long-time tails in the v elocity and stress autocorrelation functions of a dense two-dimensiona l fluid. Large systems of the order of hundred thousand particles have been investigated, performing canonical averages over an ensemble of trajectories generated on a parallel computer. We find the well-known t(-1) decay for the velocity autocorrelation function at two different densities of the;fluid, together with a faster than linear time depen dence for the mean-square displacement at long times. Although there a re indications of an asymptotically faster decay, the data are not pre cise enough to discriminate whether the decay is in agreement with the (t root 1n t)(-1) prediction of consistent mode-coupling theory or it is due to finite size effects. No evidence, within the statistical er rors, is found for a long-time tail in the stress autocorrelation func tion. This finding is in agreement with recent NEMD results [Hoover et al., Phys. Rev. E 51 (1995) 273; Gravina et al., Phys. Rev. E 52 (199 5) 6123], who find an analytical dependence of the shear viscosity upo n the shear rate with no evidence for divergence in the Green-Kubo val ue.