Intermolecular potentials for simulations of liquid imidazolium salts

Intermolecular potentials suitable for molecular dynamics or Monte Carlo simulations have been developed for dimethyl imidazolium and methyl ethyl imidazolium ions. The predicted crystal structures were compared with experimental crystal structures for chloride and PF(6)(-) salts and found to be satisfactory when the dominant electrostatic interactions were modelled by either an accurate distributed multipole description or a simplified atomic point charge model. A further simplification of using united atoms in place of methyl or methylene groups on the side chains gave a much less satisfactory reproduction of the crystal structures. Liquid dimethyl imidazolium chloride and dimethyl imidazolium PF(6)(-) were simulated using the explicit atom and united atom potentials. The local structure showed a strong preference for the chloride ions to be located in certain regions around the cation, and a similar, but less strong localization of the larger PF(6)(-). Significant differences in density and diffusion rates were found when the explicit atom model was replaced by the cheaper united atom model, showing that the latter potential is significantly poorer for modelling both the static solid and dynamic liquid simulations.