Accurate Modeling of Bromide and Iodide Hydration with Data-Driven Many-Body Potentials

<p>Ion&ndash;water interactions play a central role in determining the properties of aqueous systems in a wide range of environments. However, a quantitative understanding of how the hydration properties of ions evolve from small aqueous clusters to bulk solutions and interfaces remains elusive. Here, we introduce the second generation of data-driven many-body energy (MB-nrg) potential energy functions (PEFs) representing bromide&ndash;water and iodide&ndash;water interactions. The MB-nrg PEFs use permutationally invariant polynomials to reproduce two-body and three-body energies calculated at the coupled cluster level of theory, and implicitly represent all higher-body energies using classical many-body polarization. A systematic analysis of the hydration structure of small Br^&ndash;(H₂O)_<em>n</em> and I^&ndash;(H₂O)_<em>n</em> clusters demonstrates that the MB-nrg PEFs predict interaction energies in quantitative agreement with &ldquo;gold standard&rdquo; coupled cluster reference values. Importantly, when used in molecular dynamics simulations carried out in the isothermal&ndash;isobaric ensemble for single bromide and iodide ions in liquid water, the MB-nrg PEFs predict extended X-ray absorption fine structure (EXAFS) spectra that accurately reproduce the experimental spectra, which thus allows for characterizing the hydration structure of the two ions with a high level of confidence.</p>