The H-transfer is kinetically feasible with an activation barrier of 47 kJ/mol (48 kJ/mol for TPSS). Interestingly, the barrier is predicted significantly lower for the triplet state in ref. 37 (15-17 kJ/mol depending on the functional). A possible reason for the increase of the activation energy might be that the doubly protonated His147 is stabilized by the H-bond of H !2 to a nearby water, which was not included in the QM region in the previous study. Another interesting observation is that the electrostatic contribution from the environment (∆E ptch ) increases significantly along the reaction pathway of the proton transferring from His147 to O - 2 , 3a→4a (cf. Fig. ). Thus, ∆E ptch contributes with -33 kJ/mol (B3LYP result) to the QM/MM energy at the TS. While these electrostatic contributions are sub- The reaction is therefore clearly thermodynamically unfavorable, contradicting the previous work where the reaction was considered to be thermoneutral. Thus, both barrier and reaction energy are substantially different from those in our previous investigation with smaller QM regions. Structures and energies for the reaction 3b→4b are provided in Fig. -f and Fig. and Tab. S8 in the SI. Both of the employed functionals agree that the reaction is favorable (-187 kJ/mol for B3LYP and -158 kJ/mol for TPSS) and with small activation energy (6-8 kJ/mol depending on the functional). The Big-QM calculation does not lead to any different conclusions; here the activation energy is 10 kJ/mol and the reaction is downhill -192 kJ/mol in good agreement with the QM/MM results.