Thomas G. Lombardo and Pablo G. Debenedetti
Department of Chemical Engineering, Princeton University, Princeton, New Jersey 08544

Frank H. Stillinger
Department of Chemistry, Princeton University, Princeton, New Jersey 08544

(Received 8 August 2006; accepted 28 September 2006; published online 7 November 2006)

J. Chem. Phys. 125, 174507 (2006)

Abstract

We use molecular dynamics simulations to investigate translational and rotational diffusion in a rigid three-site model of the fragile glass former ortho-terphenyl, at 260 K £ T £ 346 K and ambient pressure. An Einstein formulation of rotational motion is presented, which supplements the commonly used Debye model. The latter is shown to break down at supercooled temperatures as the mechanism of molecular reorientation changes from small random steps to large infrequent orientational jumps. We find that the model system exhibits non-Gaussian behavior in translational and rotational motion, which strengthens upon supercooling. Examination of particle mobility reveals spatially heterogeneous dynamics in translation and rotation, with a strong spatial correlation between translationally and rotationally mobile particles. Application of the Einstein formalism to the analysis of translation-rotation decoupling results in a trend opposite to that seen in conventional approaches based on the Debye formalism, namely, an enhancement in the effective rate of rotational motion relative to translation upon supercooling.

FIG. 7. Single-molecule rotational trajectory of the unit vector u (see Fig. 1), employed in the Debye model of rotation at each investigated temperature. The total time for each trajectory is the time necessary for ácos(y(t)ñ to decay to 0.05.