XSOL, a combined integral equation (XRISM) and quantum mechanical solvation model: Free energies of hydration and applications to solvent effects on organic equilibria

A combined XRISM integral equation and quantum mechanical solvation model (XSOL) is presented and tested for the computation of free energies of hydration of organic compounds. The method features the extended reference interaction site model (XRISM) for the description of solute-solvent interactions and a quantum mechanical representation of the solute molecule. A coupled self-consistent-field procedure is used to achieve the convergence of the solvent structural reorganization and the electronic polarization of the solute molecule in solution. The present XSOL model is tested through computation of free energies of hydration of a series of organic compounds and the medium dependence of a number of organic equilibria in solution. The method complements continuum self-consistent reaction field (SCRF) methods and explicit Monte Carlo and molecular dynamics simulations, and it is much more efficient than QM/MM simulation approaches for estimating solvent effects on organic processes in aqueous solution.