for the apoenzyme (389 atoms) and 5 Å radius of Gly829 and 3 Å radius Cys493 for plus toluene and fumarate for the holoenzyme (315 atoms). The charge of the S-HL was 0 and -1, and for the B-HL was 1 and 0, respectively. All calculations were conducted for a doublet state due to the presence of a single radical. The geometry of the QM:MM models was optimized at the B3LYP/6-31g(d,p):AMBER using an electronic embedding approach which was followed by vibrational analysis introducing vibrational corrections for stationary points. The transition states of H atom transfer between Cys493 and Gly829 were localized by means of relaxed scans along the reaction coordinate (i.e., d(Cys-S-H … C rad -Gly)) followed by TS optimization using the Berny algorithm. Each stationary state preceding or following a particular TS was optimized individually, with initial geometry derived from the TS. The energy of the final stationary points was corrected with single point calculations using B-HL at B3LYP/6-311g+(2d,2p):AMBER level of theory with electronic embedding and Grimme D3 corrections for disperse interactions . The only exception from this protocol was proS intermediate of the apoenzyme (proS I apo ) for which we were able to calculate energy only at the B3LYP/6-31g(d,p):AMBER level of theory. As a result, the energy for proS I apo was approximated based on the energy difference between proS I apo and proS TS apo calculated at B-HL/B3LYP/6-31g(d,p)/D3:AMBER level of theory. The vibrational corrections were calculated at 303 K and 1 atm. and scaled with 0.9806 factor according to B3LYP/6-31g(d) correction calculated by Scott and Radom and thermal energy correction was added to all electronic energies calculated at the respective level of theory. The corrections for the transfer of the H/D atom were obtained by substitution of the mass of respective protium nuclei with an isovalue of 2, taking into consideration substitution of the transferred hydrogen atom, atom position at Gly829, both of the hydrogen atoms or none.