3.3. Protein Systems. Previously, Su and Li 107 used a heterogeneous PCM to investigate the active sites of five type-1 copper proteins (1EY5, 1ID2, 1KDI, 1PZA, and 2CAK), and we will explore the same systems in this work. Relaxed structures were not provided in Ref. 107 so we started from crystal structures. Following previous work, we first protonated these structures using the H++ web server (pH = 7.0, salinity of 0.15 M, ε in = 10, and ε out = 80), 108,109 then relaxed the geometries using the GFN2-xTB 110,111 in conjunction with a generalized Born/surface area implicit solvent model for water. The relaxed structures were trimmed prior to further relaxation at the DFT level, following the protocol in Ref. 107. A significant number of atoms were fixed (a) In 1E5Y, 1KDI, 1PZA, and 2CAK the copper is in its reduced form (Cu + ), whereas in 1ID2 it is in its oxidized form (Cu 2+ ). These charge states were specified as part of the SCF initial guess. For HetPCM calculations, a dielectric constant ε solv = 78.4 was assigned to the solvent-exposed histidine side chains, whereas the remaining atoms were assigned a value of either ε nonp = 4 or 10. The solute cavity was constructed and discretized as described in Section 3 .2.