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_{q;{*}};{C}) 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_{q;{*}};{C} 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.