The CMIRS1.0 (composite method for implicit representation of solvent, Version 1.0) model is introduced for efficacious and inexpensive computation of hydration free energies. The method collects together several disparate models designed to describe short-range dispersion, exchange, and hydrogen bonding interactions as well as long-range electrostatic interactions. All the interactions are formulated as functionals of the solute charge density. The model uses only six adjustable parameters to determine the various short-range terms. In conjunction with an isodensity criterion that uses one parameter to determine the solute cavity size and shape, the model is tested on a large database of neutral and ionic solutes in water. The mean unsigned error compared to experiment is found to be as low as 0.8 kcal/mol for neutral solutes and 2.4 kcal/mol for ionic solutes, which is comparable to or better than other analogous approaches in the literature that invoke many more fitting parameters.

Implicit solvation models provide a very efficient means to estimate solvation energies. For example, dielectric continuum models are commonly used to obtain the long-range electrostatic interactions. These may be parametrized to also include in some average manner short-range interactions such as dispersion and exchange, but it is preferable to instead develop additional implicit models specifically designed for the short-range interactions. This work proposes new models for dispersion and exchange interactions between solute and solvent by adapting approaches previously developed for treatment of gas-phase intermolecular forces. The new models are formulated in terms of the charge densities of the solutes and use only three adjustable parameters. To illustrate the performance of the models, electronic structure calculations are reported for a large number of solutes in two nonpolar solvents where short-range interactions dominate and different balances pertain between attractive dispersion and repulsive exchange contributions. After empirical optimization of the requisite parameters, it is found that the errors compared to experimental solvation free energies are only about 0.4 kcal/mol on average, which is better than previous approaches in the literature that invoke many more parameters.

A composite method for implicit representation of solvent previously developed to compute aqueous free energies of solvation is extended to accommodate the polar aprotic solvents dimethyl sulfoxide and acetonitrile. The method combines quantum mechanical calculation of the solute electronic structure with a modern dielectric continuum model for long-range electrostatic interactions with solvent and individual models for short-range interactions arising from dispersion, exchange, and hydrogen bonding. The few parameters involved are optimized to fit a standard data set of experimental solvation energies for neutrals and ions. Results are better than other models in the literature, with average errors for ions comparable to or smaller than the estimated experimental errors. Some circumstantial evidence is also obtained to support one of the competing extrathermodynamic arguments recently used to determine the solvation energies of the proton, which are needed to separate measurements of paired cation plus anion solvation energies into absolute single ion solvation energies in these solvents.

The performance in describing hydration free energies of a broad class of neutral, cationic, and anionic solutes is tested for the recently proposed FESR (Field-Extremum Short-Range) implicit solvation model for interactions between the solute and nearby water molecules, as taken in conjunction with the previously developed SS(V)PE (Surface and Simulation of Volume Polarization for Electrostatics) dielectric continuum model for long-range interactions with bulk water. The empirical FESR model mainly describes solute-water hydrogen bonding interactions by correlating them with the maximum and minimum values of the electric field produced by the solute at the surface of the cavity that excludes solvent. A preliminary report showed that, with only four adjustable parameters, the FESR model, in conjunction with SS(V)PE, can produce hydration energies comparable to the best analogous efforts in the literature that utilized many more parameters. Here, the performance of the FESR model is more fully documented in several respects. The dependence on the underlying quantum mechanical method used to treat the internal electronic structure of the solute is tested by comparing uncorrelated Hartree-Fock to correlated density functional calculations and by comparing a modest sized to a large basis set. The influence of cavity size is studied in connection with an isodensity contour construction of the cavity. The sensitivity of the results to the parameters in the FESR model is considered, and it is found that the dependence on the electric field strength is quite nonlinear, with an optimum exponent consistently in the range of 3 to 4. Overall, it is concluded that the FESR model shows considerable utility for improving the accuracy of implicit models of aqueous solvation.

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