Nonequilibrium molecular dynamics simulations of 3-methylhexane: The effect of inter- and intramolecular potential models on simulated viscosity

Nonequilibrium molecular dynamics simulations of viscosity were performed using various molecular representations of 3-methylhexane in order to study the influence of potential models on simulated viscosity. The models investigated were united atom models with fixed bond lengths and bond angles. The effect of intermolecular potential was examined by comparing results from a homogeneous (in which all -CHx groups are equivalent) model and two heterogeneous models. The effect of intramolecular potential was investigated by comparing results from three different torsional potential models. The simulations were carried out at three different densities to investigate the sensitivity of the contributions from the various models to the viscosity at different conditions. Large changes in viscosity were produced by relatively small changes in the intermolecular potential parameters of the branched methyl group. The viscosity was found to be less sensitive to the intermolecular potential parameters of the chain methyl groups and the torsional potential. Our results suggest that an accurate representation of the molecular structure and size as governed by intermolecular interactions is more important in accurate viscosity predictions than careful modeling of the intramolecular potential.