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.