It is possible that the outliers represent unusually hard samples for all models, but the more likely scenario is that the outliers are cases affected by incomplete conformational sampling or with unintended transition states (i.e., the known sources of irreducible error in RGD1). To test this hypothesis, we took the 100 lowest error testing samples and 100 highest error testing samples based on the G→ BEP model performance and re-investigated their transition states. For these 200 reactions from the RGD1 testing set, transition state searches were re-performed using 10 rather than 3 reaction conformers (i.e., the RGD1 curation protocol) and DFT-level IRC calculations were performed on the RGD1 TSs.