Studies of translational diffusion in the smectic A phase of a Gay-Berne mesogen using molecular dynamics computer simulation

Molecular dynamics computer simulations are used to determine the self-diffusion coefficients for a Gay-Berne model mesogen GB (4.4,20,1,1) in the isotropic, nematic and smectic A phases along two isobars. The values of the parallel and perpendicular diffusion coefficients, D-parallel to and D-perpendicular to, are calculated and compared in the different phases. For the phase sequence isotropic-smectic A, D-perpendicular to*greater than or equal toD(parallel to)* over the whole smectic A range with the ratio D-parallel to*/D-perpendicular to* decreasing with decreasing temperature. At a higher pressure, a nematic phase is observed between these two phases and we find that D-parallel to*>D-perpendicular to* throughout the nematic region and the inequality D-parallel to*>D-perpendicular to* remains on entering the smectic A phase. However, the ratio D-parallel to*/D-perpendicular to* decreases with decreasing temperature within the smectic A range and eventually this ratio inverts such that D-perpendicular to*>D-parallel to* at low temperatures. The temperature dependence of the parallel diffusion coefficient in the smectic A phase for this model mesogen is compared to that predicted by a theoretical model for diffusion subject to a cosine potential. (C) 2004 American Institute of Physics.