In the previous QM/MM study with a smaller QM region, reaction barriers of 111 kJ/mol (B3LYP) and 104 kJ/mol (TPSS) were found for His147 being in the HIE state. Since the HAA reaction has often been reported as the rate determining step, we have run additional calculations to confirm our results. For the HAA of conformer 6b*, the TS and the product include significant MM contribution (up to 34 kJ/mol), mainly originating from the substrate unit that is not included in the QM region. We have therefore calculated reaction energies and barriers for both conformers for the triplet state including the full substrate in the QM region (3 instead of 2 units). Unfortunately, the MM energies remain somewhat high (28 kJ/mol) for the HAA reaction involving 6b*. Still, for both conformers the energies stay essentially the same differing by less than 5 kJ/mol (see Fig. , Tab. S11, Fig. and Tab. S12). We have additionally calculated Gibbs free energies and transition states for the HAA (6b→7) using a QM-cluster approach starting from the Thus, we conclude that the electronic energies are generally a good approximation to the free energies in the case of LPMOs.