UC San Diego

Ions in aqueous solution play pivotal roles in many fundamental processes across chemical, atmospheric, and biological systems. Accurately modeling their interactions remains challenging because long-range electrostatics coexist with short-range quantum-mechanical effects. Classical force fields are computationally efficient and scalable, but their simplified functional forms often omit effects such as charge transfer, limiting their transferability beyond the conditions used for parameterization. Machine-learned interatomic potentials (MLIPs) can describe complex local environments, but many lack an explicit treatment of long-range electrostatics and therefore struggle to represent aqueous ionic systems accurately.This dissertation uses the many-body energy (MB-nrg) framework to model ion--water and ion--ion interactions while retaining physical insight through many-body energy decomposition. First, molecular dynamics simulations of aqueous halides in bulk solution are compared with corresponding results for alkali-metal ions. Fluoride forms a highly structured hydration shell and produces pronounced vibrational shifts relative to the heavier halides, highlighting the importance of many-body effects.At the air/water interface, a controlled hierarchy of MB-nrg models isolates the roles of pairwise interactions, classical polarization, and explicit short-range many-body effects. Pairwise models exclude all halides from the interface, whereas classical polarizable models overstate their interfacial propensity. The full many-body description instead suppresses adsorption for fluoride, chloride, and bromide and predicts only a shallow free-energy minimum for iodide. Energy decomposition attributes this residual preference to solvent reorganization that offsets ion--water desolvation, while an entropic penalty limits the depth of the free-energy well.Finally, finite-concentration alkali-metal halide solutions are investigated, where Coulomb interactions complicate the convergence of the many-body expansion. Many-body decomposition of aqueous clusters shows that contributions involving both ions persist to higher body orders than those in water-only or single-ion subclusters. Benchmarks of conventional force fields and MLIPs further demonstrate that accurate short-range interactions must be combined with a physically appropriate treatment of long-range electrostatics. Together, these results establish a systematic route for extending data-driven many-body models from isolated-ion hydration to ion pairing and concentrated electrolyte solutions.