Coupling and Decoupling between Translational and Rotational Dynamics in a Supercooled Molecular Liquid

We use molecular dynamics simulations to investigate the coupling and decoupling between translational and rotational dynamics in a glass-forming liquid of dumbbells. We find that the coupling between the translational (tau(*C)(q)) and rotational (tau(2)) relaxation times increases with decreasing temperature T, whereas the coupling decreases between the translational (D-t) and rotational (D-r) diffusivities. In addition, the T dependence of D-t decouples from that of 1/tau(2). We show that the decreasing coupling between D-t and D-r is only apparent due to the inadequacy of the concept of the rotational diffusion constant for describing the reorientational dynamics in the supercooled state. We also argue that the coupling between tau(*C)(q) and tau(2) and the decoupling between D-t and 1/tau(2) can be consistently understood in terms of the growing dynamic length scale.