Our calculations generally started from the [CuO] + species (1 in Fig. ). This intermediate was selected as previous calculations have shown that this is the most likely species to abstract hydrogen from the substrate, and it is readily formed from the reaction between the reduced resting state and H 2 O 2 . From these previous calculations, we know that this [CuO] + moiety is most accurately described as a Cu 2+ and an O -• radical, spin-coupled to either a triplet or an open-shell singlet. Thus, the intermediates considered here are generally expected to attain either singlet and triplet spin states, and we have always attempted to obtain both of these spin states. As previous studies showed the closed-shell singlet states to be significantly higher in energy, the singlet states were only calculated as open-shell species in this study. The open-shell singlets were obtained in a spin-unrestricted (broken symmetry) formulation and the calculations were typically initiated from the triplet-state structures (using the triplet molecular orbital coefficients as initial guess). Similarly, for calculations with def2-TZVPP the open-shell singlet calculations were always started from the triplet molecular orbital coefficients. The convergence to an open-shell singlet was always confirmed by inspection of the Mulliken spin densities (which are reported in the supporting information). We compare the obtained electronic structure of 1 to previous calculations involving the Cu(II)-oxyl (1) intermediate. However, since this has been discussed frequently in the literature, this discussion is moved to the SI.