A molecular dynamics study of the temperature dependence of static and dynamic properties of a Lennard-Jones fluid at fixed density

Joyce Noah, Stanford University

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Molecular dynamics simulations are performed for a one-component Lennard-Jones fluid at equilibrium for a fixed density and a range of temperatures. The density was that of the liquid at the triple point, and the temperatures ranged from highly supercooled (slightly above half the triple point temperature) to highly supercritical (slightly below five times the critical temperature). From these studies, we obtain a variety of structural and dynamical information. Crystalization is observed for temperatures lower than half the triple point temperature, as evidenced by thermodynamic properties (potential energy, pressure) and structural properties (radial distribution function).

The velocity autocorrelation function decays to zero from above at the longest times for high temperatures. This is the classic “long time tail,” first observed by Alder and Wainwright in 1953. As the temperature is gradually lowered into the supercooled regime, the long time behavior changes character and the decay to zero is from below, a time dependence that is indicative of the viscoelastic behavior of the fluid at low temperatures.

The most surprising result is the temperature dependence of the self-diffusion coefficient. At all temperatures in the liquid and supercooled liquid regimes, the apparent activation energy for diffusion decreases as the temperature is lowered. This behavior is different from what has been observed in simulations of more complicated liquids and different from what is typically observed experimentally for real liquids when cooled at constant pressure. This observation has a straightforward interpretation in terms of the location of the mode-coupling transition in the temperature-density plane of the Lennard-Jones fluid, and this will be an object of future research.

Abstract Author(s): Joyce Noah