Simulation of the dynamics of hard ellipsoids

We study a system of uniaxial hard ellipsoids by molecular dynamics simulations, changing both the aspect-ratio X-0 (X-0 = a/b, where a is the length of the revolution axis and b is the length of the two other axes) and the packing fraction phi. We calculate the translational < r(2)(t)> and rotational <phi(2)(t)> mean squared displacements, the translational D-trans and the rotational D-rot diffusion coefficients and the associated isodiffusivity lines in the phi - X-0 plane. For the first time, we characterize the cage effect through the logarithmic time derivative of log < r(2)(t)> and log <phi(2)(t)>. These quantities exhibit a minimum if the system is supercooled and we show that, consistently with our previous findings, for large and small X-0 values, rotations are supercooled, contrary to translations. In agreement with this scenario, while the self-intermediate scattering function exhibits stretched relaxation (i.e. glassy dynamics) only for large phi and X-0 approximate to 1, the second order orientational correlator C-2(t) show stretching only for large and small X-0 values. As further evidence of this pre-nematic order driven glass transition, we observe a decoupling of the translational and rotational dynamics, which generates an almost perpendicular crossing of the Dtrans and Drot isodiffusivity lines.