Reparameterization of an Accurate, Few-Parameter Implicit Solvation Model for Quantum Chemistry: Composite Method for Implicit Representation of Solvent, CMIRS v. 1.1

CMIRS (composite method for implicit
representation of solvent)
is a relatively new implicit solvation model that adds terms representing
solute–solvent dispersion, Pauli repulsion, and hydrogen bonding
to a continuum treatment of electrostatics. A small error in the original
implementation of the dispersion term, but one that can modify dispersion
energies by up to 8 kcal/mol in some cases, necessitates refitting
the parameters in the model, which we do here. We refer to the modified
implementation and parameter set as CMIRS v. 1.1. While the dispersion
energies change in nontrivial ways, an increase in the attractive
dispersion term in the new implementation is largely offset by an
increase in the Pauli repulsion during the fitting process, such that
overall statistical errors are virtually unchanged with respect to
v. 1.0 of the model, for a large database of experimental solvation
free energies for molecules and ions. Overall, we obtain mean unsigned
errors of <0.7 kcal/mol when the solvent is cyclohexane or benzene,
<1.5 kcal/mol for water, and <2.8 kcal/mol for dimethyl sulfoxide
and acetonitrile, despite using no more than five empirical parameters
per solvent. For the important but difficult case of ionic solutes
in water, mean unsigned errors are <2.9 kcal/mol.