We further investigated the effect of H2O2 binding on the electronic structure. H2O2 was added to Cu(I) and Cu(I)-Cell-Cl -, replacing either the equatorial water or chloride (note that with 4.03 Å and 3.84 Å respective distances to copper, neither the water nor the chloride is in coordination distance). In all structures, coordinates of H2O2 were freely optimized together with the hydrogen atoms, employing the procedure Table : Selected distances for the structures shown in Figure . Distances with only one atom are between the indicated atom and copper. For the equatorial and axial water ligands, the distance to oxygen is reported. In the case of two oxygen atoms in H2O2, only the distance to the oxygen closer to copper is reported, while the distance between the two oxygen atoms is given in the last column. A more extensive list of distances and angles for all investigated structures is given in the SI (Table ).