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 C₁⁽ˢ⁾(t), C₂⁽ˢ⁾(t), and C₁(t) show at low temperatures a power law with the same critical temperature Tc, which is also identical to the critical temperature for the translational degrees of freedom. In contrast to MCT, we find, however, that for these correlators the time-temperature superposition principle does not hold well and also the critical exponent {γ} depends on the correlator. For Cₗ⁽ˢ⁾ with l=3,,6 this principle does hold. We also study the temperature dependence of the rotational diffusion constant Dᵣ and demonstrate that at high temperatures Dᵣ 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 Cₗ⁽ˢ⁾) and Dᵣ. Finally, we present results that show that at low temperatures 180^∘{} flips of the molecule are an important component of the relaxation dynamics for the orientational degrees of freedom.