When testing the PM6-HCON and PM6-HCONS radii on pKa prediction (see below) we found that several solution-phase geometry optimizations failed because some X-H bonds broke and the proton started moving into the solvent. This indicated that the H radius might be too small, leading to overpolarization of the H atom due to the solvent, and we tested several larger H radii for PM6-HCONS since the H radius was larger than for PM6-HCON. We found that the overpolarization problem disappeared with a H radius of 0.6 Å and that using this value has a negligible 0.1 kcal/mol e ect of the accuracy of the solvation energies (HCONSh in Table ). Thus, we use PM6-HCONSh radii set going forward. . HCO indicates that hydrogen, carbon, and oxygen radii parameters are changed (i.e. second and sixth entry in Table . The reference set is split into full set (black), neutral (green), cations (red), and anions (blue). From the results discussed above and presented in Table , optimizing for atom types beyond HCON showed no significant increase in accuracy for either PM3 or PM6. Therefore the same procedure (optimizing HCON radii) was applied to DFTB. For DFTB the RMSE values of the full set of molecules, the subset of cation and the subset of anions goes from 4.1, 4.9 and 6.7 kcal/mol to 3.3, 2.9 and 4.4 kcal/mol, respectively.