Application of static charge transfer within an ionic-liquid force field and its effect on structure and dynamics

The effects of linear scaling of the atomic charges of a reference potential on the structure, dynamics, and energetics of the ionic liquid 1,3-dimethylimidazolium chloride are investigated. Diffusion coefficients that span over four orders of magnitude are observed between the original model and a scaled model in which the ionic charges are ± 0.5 e. While the three-dimensional structure of the liquid is less affected, the partial radial distribution functions change markedly - with the positive result that for ionic charges of ± 0.7 e, an excellent agreement is observed with ab initio molecular dynamics data. Cohesive energy densities cal culated from these partial-charge models are also in better agreement with those calculated from the ab initio data. We postulate that ionic-liquid models in which the ionic charges are assumed to be ± 1 e overestimate the intermodular attractions between ions, which results in overstructuring, slow dynamics, and increased cohesive energy densities. The use of scaled-charge sets may be of benefit in the simulation of these systems - especially when looking at properties beyond liquid structure - thus providing an alternative to computationally expensive polarisable force fields.