For PM6, the RMSE ú for HCO (3.2 kcal/mol) is about 1 kcal/mol lower than for the other radii set. However, the RMSE values computed using all molecules are very similar with a range of 3.6 to 3.8 kcal/mol, with HCO and HCON tied for first place. HCO does lead to the lowest error for neutral molecules and anions, but also an RMSE that is significantly larger for cations (6.2 kcal/mol), where PM6-HCONS performs best.