i‑TTM Model for Ab Initio-Based Ion–Water Interaction Potentials. 1. Halide–Water Potential Energy Functions

New potential energy functions (i-TTM) describing the interactions between halide ions and water molecules are reported. The i-TTM potentials are derived from fits to electronic structure data and include an explicit treatment of two-body repulsion, electrostatics, and dispersion energy. Many-body effects are represented through classical polarization within an extended Thole-type model. By construction, the i-TTM potentials are compatible with the flexible and fully ab initio MB-pol potential, which has recently been shown to accurately predict the properties of water from the gas to the condensed phase. The accuracy of the i-TTM potentials is assessed through extensive comparisons with CCSD­(T)-F12, DF-MP2, and DFT data as well as with results obtained with common polarizable force fields for X^–(H₂O)_n clusters with X^– = F^–, Cl^–, Br^–, and I^–, and n = 1–8. By construction, the new i-TTM potentials will enable direct simulations of vibrational spectra of halide-water systems from clusters to bulk and interfaces.