Similar to the N2O activation step, a significant difference in the value of the energy barrier is again observed between PBE and HSE06, with values of 55 kJ/mol from HSE06 and 125 kJ/mol from PBE. However, the sign of the difference is inverted, meaning that the barrier is higher in PBE than in HSE06 for the C-H bond breaking (see also Figure ). Since the difference between the two functionals is inverted for the C-H bond breaking step compared to the N-O bond breaking step, the overall reaction energy of the entire catalytic cycle does not differ as strongly between PBE and HSE06 as the two individual reaction steps. However, the step with the highest energy barrier is the formation of the iron-oxo species according to the HSE06 energy profile, whereas the step with the highest energy barrier is the C-H activation according to the PBE profile. The higher energy barrier for the N-O step compared to the C-H bond breaking is in line with previous PBE, PBE+U and M06-L based studies by Snurr and co-workers. The radical nature of the methyl 1 TS CH structure can be clearly seen in the visualization of the magnetization density on the right side of Figure . One unpaired electron is located on the methyl, and the remaining three unpaired electrons of the same spin are located around the iron center. As Feldt et al. suggest that the CH3 radical structure can also be in an antiferromagnetically coupled (AFM) state with an S=2.5 iron center and an S=0.5 CH3 radical, we investigated this possibility. Our HSE06 SP calculations on the 1 TS CH structure, however, favor the high spin S=2 state over the AFM S=2 state by 20.3 kJ/mol. The weak methyl-iron interaction (the methyl radical is only physisorbed, see above) could be useful for maintaining an active catalyst, as a strong methyl-iron bond can be expected to increase the creation of unwanted carbene and water, which poison the catalyst.