Charge scaling parameter evaluation for multivalent ionic liquids with fixed point charge force fields

Within the context of molecular simulations, one of the key force field parameters is the value of the partial charges on different atom sites. Due to the influential role of electrostatics in the behavior of ionic liquids (ILs), the assignment of partial charges is a particularly important aspect of the force field to evaluate. In this context, results from quantum chemical calculations suggest that the net charge of monovalent cations and anions can often be less than unity (i.e., +/- 1). Such effects can be captured using a polarizable force field, but the computational cost can often be prohibitive. Thus, charge scaling approaches can provide a practical strategy for improving the predictive accuracy of non-polarizable force fields. Here, we use both simulations and experiments to study the effect of charge scaling on the density predictions of multivalent ILs solubilized in water and acetonitrile. Our results suggest that more significant charge scaling is necessary when modeling multivalent ILs, versus what has been previously found for monovalent ILs.