Moving to the reduced Cu(I) and Cu(I)-Cell structures, we see from Figure (and Figure ) that the orbitals of copper d-character are now in both cases close to the frontier -and that the binding of the substrate does not change this fact. The Cu(I) HOMO-1 is the dx2-y2-orbital shown in Figure . Consistent with Lim et al. , we observe that the dx2-y2-orbital in the Cu(I) structure is isolated and 1.3-1.6 eV higher in energy than the other copper d-orbitals (see Table ). However, we also observe that after the substrate binds, the copper orbitals of d-character are closer in energy (see Cu(I)-Cell in Figure and S10): The energy difference between the dx2-y2-orbital and the next lower Cu d-orbital is reduced more than three-fold upon substrate binding, from 1.3 eV in the substrate-free Cu(I) structure to 0.4 eV in the substrate-bound Cu(I)-Cell structure. This was not seen by Lim et al. since they did not include a substrate. However, they did investigate the effect of the co-substrate binding (in this case H2O2) on these energy differences, and we therefore further analyse these differences in more detail in the following Section. Before explicitly including an oxidizing species, we note that the proposal with an isolated dx2-y2-orbital high in energy was also suggested by Courtade et al. They argued that substrate binding induced the relative increase of the dx2-y2-orbital. Their study was based on EPR spectra measured for BlAA10 with and without chitin and thus inherently based on the Cu(II) states.