Ab initio extension of the AMOEBA polarizable force field to Fe ²⁺

We extend the AMOEBA polarizable molecular mechanics force field to the Fe²⁺ cation in its singlet, triplet, and quintet spin states. Required parameters are obtained either directly from first principles calculations or optimized so as to reproduce corresponding interaction energy components in a hexaaquo environment derived from quantum mechanical energy decomposition analyses. We assess the importance of the damping of point-dipole polarization at short distance as well as the influence of charge-transfer for metal-water interactions in hydrated Fe²⁺; this analysis informs the selection of model systems employed for parametrization. We validate our final Fe²⁺ model through comparison of molecular dynamics (MD) simulations to available experimental data for aqueous ferrous ion in its quintet electronic ground state.