  We use molecular dynamics simulations to investigate translational and
rotational diffusion in a rigid three-site model of the fragile glass former
ortho-terphenyl, at 260 K < T < 346 K and ambient pressure. An Einstein
formulation of rotational motion is presented, which supplements the
commonly-used Debye model. The latter is shown to break down at supercooled
temperatures as the mechanism of molecular reorientation changes from small
random steps to large infrequent orientational jumps. We find that the model
system exhibits non-Gaussian behavior in translational and rotational motion,
which strengthens upon supercooling. Examination of particle mobility reveals
spatially heterogeneous dynamics in translation and rotation, with a strong
spatial correlation between translationally and rotationally mobile particles.
Application of the Einstein formalism to the analysis of translation-rotation
decoupling results in a trend opposite to that seen in conventional approaches
based on the Debye formalism, namely an enhancement in the effective rate of
rotational motion relative to translation upon supercooling.
