With the exception of four model reactions that failed to find an intended TS (Fig. ), comparisons of activation energies between model reactions and actual reactions are made separately for b2f2 and b3f3 reactions (Fig. ). The mean signed error (MSE) and mean absolute error (MAE) of b2f2 and b3f3 reactions are -0.30/5.70 and 1.20/3.43 kcal/mol, respectively (Fig. ). The larger errors associated with b2f2 reactions arise mainly from reactions with topology changes that are overrepresented in the b2f2 class relative to the b3f3 class, which tends to preserve rings and molecularity during reactions. The deviation distribution is further divided into four classes according to the number of heavy atoms reduced by the model reaction (Fig. ). The model reactions with no heavy atom reduction but a topology change (∆Topology in Fig. ) between the model reaction and parent reaction have the highest MAE of 7.85 kcal/mol, followed by model reactions with reduction of 1, 2 and more than 2 heavy atoms relative to the parent reaction, with MAE of 4.82, 5.82, 5.99 kcal/mol, respectively. The high deviation in the first case is because the model reaction does not preserve the bond between next-nearest atoms, thus converting a unimolecular reactant into a bimolecular or a cyclic reactant into linear (i.e. change in topology). The higher flexibility of the resulting bimolecular and linear model reaction leads to a systematic reduction in the activation energy of around 6 kcal/mol. Since all 21 such reactions are b2f2 reactions, this also explains the higher deviation of b2f2 reactions.