Fig. 1. QM/MM (UB3LYP/B2//B1) energy profiles (in kcal mol^–1) for Cpd I formation from Cpd 0 in the T268A mutant: (a) the coupling-I pathway for the major conformer leads to Cpd I formation and (b) protonation of Cpd 0 in the minor conformer leads to Fe(H₂O₂) formation. The energy barriers include ZPE corrections. Sub 1 is included in the MM part, and MD simulations show that it is located too far from Fe(H₂O₂) to barricade H₂O₂ against detachment (ESI, Fig. S3†). Thus, Cpd I formation from Fe(H₂O₂) (coupling-II⁷) can be ruled out in the T268A mutant of BM3.