The best-performing models included four empirical methods and one QM-based model. These five methods were able to predict the acidity constants of the challenge compounds to within 1 pK a unit. In fact, while most empirical models -except for the DL and two of the five QSPR/ML approaches -were able to predict the acidity constants to within about 1.5 pK a units, the range of predictions was much wider for the QM-based models.