This section examines the outliers associated with the held-out test set (20 %; 145 compounds; 671 hydricities) for the best regression model, see Figure and Table . The outliers were checked to see if the QM calculations failed or if the conformations were wrong. However, this was not the case. For compounds ibond193, comp294, shen50, and ibond301, the high difference in hydricities between QM and ML occurs at the allyl-like cations. The ML model consequently predicts these cations to be less stable than the QM method. This is likely due to the extra stability gained through resonance effects that the ML model cannot describe. Resonance or inductive effects from electron-donating-or electron-withdrawing groups (EDG, EWG) seem to be a factor for the outliers. Another example is for comp227, where the ML model predicts the lowest hydricity site to be 8 (QM: 109.39; ML 102.26), but the QM calculations predict the lowest hydricity site to be 0 (QM: 92.71; ML 112.51). The carbocation at site 8 has an antiaromatic character and is thus destabilized.