Bulk viscosity of model fluids. A comparison of equilibrium and nonequilibrium molecular dynamics results

Abstract

We compare accurate equilibrium and nonequilibrium molecular dynamics calculations for the bulk viscosity of model fluids. Results for the ‘‘soft-sphere fluid’’ as well as for the ‘‘Lennard-Jones fluid’’ are considered. Particle number dependence and errors due to the cutoff of the potential are investigated. The accuracy of the correlation functions is discussed. Main results are the following: (i) For the soft-sphere fluid, good agreement between the two MD methods exists. (ii) For the Lennard-Jones fluid, similar good agreement can be achieved when particle numbers larger than 108 are used. The reason for this is a significant cutoff radius dependence of the bulk viscosity results, when the potential is cut at a separation smaller than 2.5 σ. Such a cutoff radius dependence does not occur for the shear viscosity. (iii) The nonequilibrium MD data given by Heyes for the soft-sphere fluid are incorrect. (iv) The Green–Kubo correlation function for the soft-sphere fluid oscillates at very high density. Such a behavior has not yet been observed for Lennard-Jones systems.