Interestingly, the HG-optimized radii actually gave better accuracy for the test set than for the training set, with RMSE lower by 1.0 kcal/mol and R 2 increased by about 0.3. Closer examination reveals that the RMSE of the cyclodextrins and cucurbit[n]urils are similar between the training and tests, but the RMSE of the octa-acids falls from 4.4 kcal/mol in the training set to 1.8 kcal/mol in the test set. We observed that the octa-acid systems in the training set that are overestimated (more negative) by at least 4 kcal/mol when using the HG-optimized radii (Figure and Table ) have guest molecules with cationic nitrogens (Figure ). We suspect that the reduction in the nitrogen GB radius, which greatly improves ∆G b for cucurbit[n]urils, slightly worsens the accuracy for octa-acids with ammonium guests, and that this may explain the fact that the overall correlation with experiment is better for the test set than the training set. and, and the error statistics are summarized in Table .