Dynamics of the rotational degrees of freedom in a supercooled liquid of diatomic molecules

Using molecular-dynamics computer simulations, we investigate the dynamics of the rotational degrees of freedom in a supercooled system composed of rigid, diatomic molecules. The interaction between the molecules is given by the sum of interaction-site potentials of the Lennard-Jones type. In agreement with mode-coupling theory (MCT), we find that the relaxation times of the orientational time correlation functions C-1((s))(t), C-2((s))(t), and C-1(t) show at low temperatures a power law with the same critical temperature T-c, which is also identical to the critical temperature for the translational degrees of freedom. In contrast to MCT, we find, however, that for these cor-relators the time-temperature superposition principle does not hold well and also the critical exponent gamma depends on the correlator. For C-1((s)) with l = 3,...,6 this principle does hold. We also study the temperature dependence of the rotational diffusion constant D-r and demonstrate that at high temperatures D-r is proportional to the translational diffusion constant D and when the system starts to become supercooled the former shows an Arrhenius behavior, whereas the latter exhibits a power-law dependence. We discuss the origin for the difference in the temperature dependence of D (or the relaxation times of C-1((s))) and D-r. Finally, we present results that show that at low temperatures 180 degrees flips of the molecule art an important component of the relaxation dynamics for the orientational degrees of freedom.