The Potential Energy Landscape and Mechanisms of Diffusion in Liquids
Abstract
The mechanism of diffusion in supercooled liquids is investigated from the potential energy landscape point of view, with emphasis on the crossover from high- to low-T dynamics. Molecular dynamics simulations with a time dependent mapping to the associated local mininum or inherent structure (IS) are performed on unit-density Lennard-Jones (LJ). New dynamical quantities introduced include r2is(t), the mean-square displacement (MSD) within a basin of attraction of an IS, R2(t), the MSD of the IS itself, and gloc(t) the mean waiting time in a cooperative region. At intermediate T, r2is(t) posesses an interval of linear t-dependence allowing calculation of an intrabasin diffusion constant Dis. Near Tc diffusion is intrabasin dominated with D = Dis. Below Tc the local waiting time tauloc exceeds the time, taupl, needed for the system to explore the basin, indicating the action of barriers. The distinction between motion among the IS below Tc and saddle, or border dynamics above Tc is discussed.
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