The occurrence of multiple connectivity changes within a single step is also a contributing factor to errors in the activation energy estimation. As detailed in Section 2, each atom that undergoes a change in connectivity is fixed during minimization to preserve distances and angles of transitional structures. Therefore, the greater the number of connectivity changes, the smaller the number of atoms whose positions can be optimized. This influences the quality of the activation energy estimation, which can be seen in R17 and R27. R27 is a concerted dissociation and rearrangement reaction involving four heavy atoms, which leads to energy overestimation by 23.8 kcal mol -1 . The β-scission reaction R17 changes connectivities of five atoms leading to a even larger energy overestimation of 70.8 kcal mol -1 .