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Novel Computational Probes of Diffusion in Supercooled Liquids and Their Application to Rotation-Translation Decoupling in O-Terphenyl

Thomas Lombardo1, M. Scott Shell2, Frank H. Stillinger3, and Pablo G. Debenedetti1. (1) Chemical Engineering, Princeton University, Princeton, NJ 08544, (2) Pharmaceutical Chemistry, University of California San Francisco, 600 16th Street, Box 2240, San Francisco, CA 94143, (3) Chemistry, Princeton University, Princeton, NJ 08544

Recently we proposed a reformulation of the diffusion constant [1] and applied the formalism to study dynamic heterogeneity and non-Gaussian behavior in a binary Lennard-Jones liquid mixture [2]. We discuss this new approach and apply it to study translational and rotational diffusion in the Lewis-Wahnström model of o-terphenyl [3]. We find that the extent of decoupling between rotational and translational motion upon supercooling is sensitively dependent on the method used to calculate rotational diffusion rates. In particular, the traditional view of rotational diffusion as Brownian motion on a sphere leads, at sufficiently low temperatures, to an effective increase in translational diffusion relative to rotational motion. Use of an Einstein expression for rotational motion yields the opposite trend, namely an effective increase in the ratio of rotational to translational diffusivity upon supercooling. We discuss the implications of these results as to the interpretation of experimental data on rotation-translation decoupling in supercooled liquids.

[1] Stillinger, F.H., and Debenedetti, P.G., J. Phys. Chem. B, 109, 6604, 2005. [2] Shell, M.S., Debenedetti, P.G., and Stillinger, F.H., J.Phys.: Condens. Matter, 17, S4035, 2005. [3] Lewis, L.J., and Wahnström, G., Phys. Rev. E, 50, 3865, 1994.