  We find in a model system of thermotropic liquid crystals that the
translational diffusion coefficient parallel to the director $D_{\parallel}$
first increases and then decreases as temperature drops through the nematic
phase, and this reversal occurs where the smectic order parameter of the
underlying inherent structures becomes significant for the first time. We
argue, based on an energy landscape analysis, that the coupling between
orientational and translational order can play a role in inducing the
non-monotonic temperature behavior of $D_{\parallel}$. Such a view is likely to
form the foundation of a theoretical framework to explain the anisotropic
translation diffusion.
