Simulations of imidazolium ionic liquids: when does the cation charge distribution matter?

We compare the properties of models of liquids and crystals constructed from a number of intermolecular potentials for dimethylimidazolium chloride [dmim][Cl]. The force fields differ in the charge distribution in the cation but all include short range interactions which determine the shape of the cation. In addition to 'realistic' models intended for simulation of [dmim][Cl] we take two extreme 'unrealistic' models in which the cation charge is localized on the ring atoms or at the ring centre in order to study the effects of the cation charge distribution. The effects of polarizability are investigated by using shell models for the chloride ion. We find that, while equilibrium properties such as energetics, crystal structure, liquid structure and charge screening depend on the charge distribution in the cation but are little affected by including polarizability, dynamical properties such as diffusion are strongly affected by polarizability.