range supra - additive slowdown of water rotation †

The anion-cation interactions are optimized against the experimentally determined solution activity derivative at 2.5 m. This property directly depends on ion-ion interactions, thus making it an excellent target for parameterization. We opt for this indirect approach because directly parameterizing anion-cation interactions from quantum mechanical calculations is not feasible, as described below. The activity derivative can be easily calculated in simulations using the Kirkwood-Buff (KB) solution theory1,2, as summarized below. Approaches based on the KB theory have been successfully used to optimize anion-cation interaction parameters involving non-polarizable models of simple monovalent and divalent ions3–7 or between water and neutral solutes7–9, lending confidence to this choice.