Some conclusions regarding the predictions of tautomeric equilibria in solution based on the SAMPL2 challenge

The COSMO-RS method, a combination of the quantum chemical dielectric continuum solvation model COSMO with a COSMO based statistical thermodynamics of surface interactions, has been used in its COSMOtherm implementation for the direct, blind prediction of tautomeric equilibria within the SAMPL2 challenge. Since the quantum chemical level underlying COSMOtherm, i.e. BP/TZVP DFT-calculations, is known to be of limited accuracy with respect to reaction energies, we tested MP2 reaction energy corrections in addition. As expected, the straight application of the latest version of COSMOtherm yielded a poor predictive accuracy of approximately 4 kcal/mol (RMSE) for the eight compounds of the blind prediction data set, and the MP2-corrected predictions reduced the average error considerably to approximately 1.2 kcal/mol. But a more detailed analysis shows that this improvement is not systematic and mostly a lucky coincidence on the small data set. The systematic results of COSMOtherm allow for an efficient empirical correction with an RMSE of 0.61 kcal/mol. This allows for systematic predictions for the most important case of generalized keto-enol tautomerism.