In summary, we find that optimizing the SMD radii in combination with either PM3 or PM6 gives appreciable decreases in RMSE accuracy ¥ 7 kcal/mol in both cases compared to the reference results. We also find that no significant improvement is obtained when attempting to optimize the radii of other atoms than what was done with HCON subset of radii. For this subset, DFTB also saw an increase in accuracy for ions of about ¥ 6 kcal/mol. Because of this, we found the best choice was change the radii for hydrogen, carbon, oxygen and nitrogen (HCON) for PM3 and DFTB methods, For PM6 we optimized including sulfur and changed the radius of hydrogen to 0.6 Å to remove overpolarization problems. These methods are denoted as SMD † going forward.