Modeling ionic liquids using a systematic all-atom force field

A new force field for the molecular modeling of ionic liquids of the dialkylimidazolium cation family was constructed. The model is based on the OPLS-AA/AMBER framework. Ab initio calculations were performed to obtain several terms in the force field not yet defined in the literature. These include torsion energy profiles and distributions of atomic charges that blend smoothly with the OPLS-AA specification for alkyl chains. Validation was carried out by comparing simulated and experimental data on fourteen different salts, comprising three types of anion and five lengths of alkyl chain, in both the crystalline and liquid phases. The present model can be regarded as a step toward a general force field for ionic liquids of the imidazolium cation family that was built in a systematic way, is easily integrated with OPLS-AA/AMBER, and is transferable between different combinations of cation-anion.