  Unlike crystalline solids or ideal gases, transport properties remain
difficult to describe from a microscopic point of view in liquids, whose
dynamics result from complex energetic and entropic contributions at the atomic
scale. Two scenarios are generally proposed: one represents the dynamics in a
fluid as a series of energy barrier crossings, leading to Arrhenius-like laws,
while the other assumes that atoms rearrange themselves by collisions, as
exemplified by the free volume model. To assess the validity of these two
views, we computed, using molecular dynamics simulations, the transport
properties of the Lennard-Jones fluid and tested to what extent the Arrhenius
equation and the free volume model describe the temperature dependence of the
viscosity and of the diffusion coefficient at fixed pressure. Although both
models reproduce the simulation results over a wide range of pressure and
temperature covering the liquid and supercritical states of the Lennard-Jones
fluid, we found that the parameters of the free volume model can be estimated
directly from local structural parameters, also obtained in the simulations.
This consistency of the results gives more credibility to the free volume
description of transport properties in liquids.
