Based on this electronic-structure methodology, we optimized the triplet (T 1 ) and singlet (S 0 ) minima as well as the MECP, which is defined as the global minimum on the crossing seam between the triplet and singlet states. We present the molecular structures in the Supporting Information. Using steepest descent in mass-weighted coordinates, we further located the minimum-energy pathways (MEPs), also known as Fukui paths, which connect the reactant and product minima via the MECP. These pathways are shown in Fig. (a) projected onto two coordinates which measure the deviation from planarity and mirror symmetry and are selected to illustrate the key characteristics of the reaction. As seen in Fig. , the activation energy required to reach the MECP is 0.494 eV and the overall reaction is highly exothermic with a thermodynamic driving force of 1.930 eV. As both the reactant and product minima are located on the same side of the crossing seam, the reaction is said to be in the