In both substrate-bound structures (Cu(I)-Cell-Cl -and Cu(II)-Cell-Cl -) a chloride anion is present (it coordinates to copper in the Cu(II)-Cell-Cl -structure). The results for the structures with chloride are discussed in Section S2 in the SI. We also substituted chloride with water, keeping the oxygen atom fixed at the position of the chloride atom. The structures with water are labelled Cu(I)-Cell and Cu(II)-Cell (see Figure ) and their hydrogen positions were optimized as described above.