TRANSLATIONAL AND ROTATIONAL MOTION IN MOLECULAR LIQUIDS - A COMPUTER-SIMULATION STUDY OF LENNARD-JONES ELLIPSOIDS

In order to understand the translational and rotational motion in dens e molecular liquids, detailed molecular dynamics simulations of Lennar d-Jones ellipsoids have been carried out for three different values of the aspect ratio kappa. For ellipsoids with an aspect ratio equal to 2, the product of the translational diffusion coefficient (D-T) and th e average orientational correlation time of the l-th rank harmonics (t au(lR)), converges to a nearly constant value at high density. Surpris ingly, this density independent value of D-T tau(lR) is within 5% of t he hydrodynamic prediction with the slip boundary condition. This is d espite the fact that both D-T and tau(lR) themselves change nearly by an order of magnitude in the density range considered, and the rotatio nal correlation function itself is strongly nonexponential. For small aspect ratios (kappa less than or equal to 1.5), the rotational correl ation function remains largely Gaussian even at a very large density, while for a large aspect ratio (kappa greater than or equal to 3), the transition to the nematic liquid-crystalline phase precludes the hydr odynamic regime. Thus, the rotational dynamics of ellipsoids show grea t sensitivity to the aspect ratio. At low density, tau(lR) goes throug h a minimum value, indicating the role of interactions in enhancing th e rate of orientational relaxation. (C) 1997 American Institute of Phy sics. [S0021-9606(97)50142-5].