Benchmarking Different QM Levels for Usage with COSMO-RS

The COSMO-RS method is
an established method for the prediction of fluid phase properties
such as activity coefficients, liquid–liquid equilibria, and
free energy of solvation. It is also frequently used in quantum chemistry-based
chemical reaction modeling to predict the solvation contribution to
the reactions. The COSMOtherm software, which features
the currently most advanced implementation of COSMO-RS, is based on
quantum chemical COSMO calculations using the BP functional with the
def2-TZVPD basis set. As the accuracy of COSMO-RS depends on the accuracy
of the underlying quantum chemical (QC) calculation, it is important
to validate the currently used level against other common, presumably
superior, approaches such as the more recently developed M06-2x hybrid
density functional or wave function methods such as MP2. As compared
to other applications where the electronic energy is the most important
result of the QC calculation, the COSMO-RS method has a much higher
dependence on the molecular polarity and thus the electron density
distribution. We find that MP2, PBE0, and M06-2x perform slightly
worse in fully reparametrized COSMO-RS with respect to the prediction
of experimentally measured properties like pKa or logP. Although MP2 was reported to yield better polarities
than most DFT functionals for spin unpolarized molecules, this theoretical
advantage does not manifest in a practical benefit for the prediction
of thermodynamic properties with a refitted COSMO-RS parameter set.
Other pure DFT functionals such as PBE or TPSS can be used instead
of BP, but again, no practical advantage is expected as they yield
extremely similar polarities to the original BP calculations.