Behavior of collective time correlation functions in liquids composed of polyatomic molecules

Abstract

Recent molecular dynamics (MD) calculations for molecular liquids showed that the time correlation functions (TCFs) for thermal transport coefficients have a characteristic ‘‘chair form.’’ For dense liquid states, a weak positive branch of the TCFs of the shear and the bulk viscosity was found. This ‘‘long‐time’’ behavior of the TCFs appeared to be of particular significance in model liquids of molecules of nonglobular shape, as n‐butane, nitrogen or benzene. Molecular liquids of molecules of more spherical shape displayed this time tail of the TCF only to a much smaller extent. The origin of this microscopic long‐time behavior of the TCFs governing the viscosity coefficients was unknown, particularly, due to the fact that it appeared also for states considerably far from the triple point state. In the present MD study we investigate in detail the behavior of the TCFs for the shear and the bulk viscosity in a liquid composed of diatomic molecules. Molecule number and potential cutoff effects are discussed for the two‐center Lennard‐Jones liquid. The long‐time behavior of the TCF is explained by long‐lived orientational correlations measured in terms of the reorientational CF of lowest even order. This explanation is corroborated by additional calculations for the dumbbell fluid at high density. Because of the occurrence of a long‐ranged weak positive branch of the TCFs for the viscosity, accurate determination of the viscosity coefficients in liquids of considerably nonisotropic molecules requires a few hundred thousand MD integration steps. On the other hand, systems of a small number of molecules give TCFs not significantly different from those obtained with larger systems.