Methods To Improve the Calculations of Solvation Model Density Solvation Free Energies and Associated Aqueous pKa Values: Comparison between Choosing an Optimal Theoretical Level, Solute Cavity Scaling, and Using Explicit Solvent Molecules.

Many approaches have been used to improve the accuracy of implicit solvent models including solute cavity scaling, introducing explicit solvent molecules, and changing the level of theory for the solvation calculations. Here, we compare these strategies using a large test set of aqueous pK_a values for amines, nucleobases, carboxylic acids, thiols, peptide carbon acids, alcohols, and anilines for the specific case of solvation model density (SMD) within the framework of a thermodynamic cycle in which the gas-phase component is consistently calculated via the accurate CBS-QB3 method. We show that the choice of theoretical level for solvation energies should be based on the original parameterization of the solvent model, with separate levels of theory for the solvation energies of neutrals, anions, or cations, outperforming the best compromise level of theory. However, when explicit solvent molecules are introduced, a higher level of theory is needed to describe the solute-solvent interactions. For the systems studied here, explicit solvation improved the results for acids (and hence anions) but not for bases, for which results deteriorated. Importantly, we find that solute cavity scaling does not significantly improve the SMD results for the CHNO compounds tested when the correct theoretical level is employed and explicit solvent effects are correctly treated.