Free Energies of Solvation with Surface, Volume, and Local Electrostatic Effects and Atomic Surface Tensions to Represent the First Solvation Shell

Liu, J.; Kelly, C. P.; Goren, A. C.; Marenich, A. V.; Cramer, C. J.;
Truhlar, D. G.; Zhan, C.-G.

* J. Chem. Theory Comput.*
**2010**, *6*, 1109.
(doi:10.1021/ct100025j).

Building on the SVPE (surface and volume polarization for electrostatics)
model for electrostatic contributions to the free energy of solvation with
explicit consideration of both surface and volume polarization effects, on
the SM*x* approach to including first-solvation-shell contributions, and on
the linear relationship between the electric field and short-range
electrostatic contributions found by Chipman, we have developed a new
method for computing absolute aqueous solvation free energies by combining
the SVPE method with semiempirical terms that account for effects beyond
bulk electrostatics. The new method is called SMVLE, and the elements it
contains are denoted by SVPE-CDSL where SVPE denotes accounting for bulk
electrostatic interactions between solute and solvent with both surface and
volume contributions, CDS denotes the inclusion of solvent cavitation,
changes in dispersion energy, and possible changes in local solvent
structure by a semiempirical term utilizing geometry-dependent atomic
surface tensions as implemented in SM*x* models, and L represents the local
electrostatic effect derived from the outward-directed normal electric
field on the cavity surface. The semiempirical CDS and L terms together
represent the deviation of short-range contributions to the free energy of
solvation from those accounted for by the SVPE term based on the bulk
solvent dielectric constant. A solute training set containing a broad range
of molecules used previously in the development of SM6 is used here for
SMVLE model calibration. The aqueous solvation free energies predicted by
the parameterized SMVLE model correlate exceedingly well with experimental
values. The square of the correlation coefficient is 0.9949 and the slope
is 1.0079. Comparison of the final SMVLE model against the earlier
SM*x*
solvation model shows that the parameterized SMVLE model not only yields
good accuracy for neutrals but also significantly increases the accuracy
for ions, making it the best implicit solvation model to date for aqueous
solvation free energies of ions. The semiempirical terms associated with
the outward-directed electric field account in a physical way for the
improvement in the predictive accuracy for ions. The SMVLE method greatly
decreases the need to include explicit water molecules for accurate
modeling of solvation free energies of ions.