The dimeric CUA Site found in cytochrome c oxidase and nitrous oxide reductase has been studied with the density functional B3LYP method. We have optimized the structure of the realistic (Im)(S(CH3)(2))-Cu(SCH3)(2)Cu(Im)(CH3CONHCH3) model in the fully reduced. mixed-valence, and fully oxidized states. The optimized structures are very similar to crystal structures of the protein, which shows that the protein does not strain the site significantly. Instead, inorganic model complexes of the protein site are strained by the macrocyclic connections between the ligand models. For the mixed-valence (Cu-I + Cu-II) state, two distinct equilibrium structures were found, one with a short Cu-Cu distance, 248 pm, similar to the protein structure, and one with a longer distance, 310 pm, similar to what is found in inorganic models. In the first state, the impaired electron is delocalized over both copper ions, whereas in the latter, it is more localized to one of the ions. The two states are nearly degenerate. The potential energy surfaces for the Cu-Cu, Cu-S-Met and Cu-O interactions are extremely flat. In fact. all three distances can be varied between 230 and 310 pm at an expense in energy of less than 8 kJ/mol, which explains the large variation observed in crystal structures for these interactions. Inclusion of solvation effects does not change this significantly. Therefore, we can conclude that a variation in these distances can change the reduction potential of the CUA site by at most 100 mV. The model complex has a reorganization energy of 43 kJ/mol, 20 kJ/mol lower than for a monomeric blue-copper site. This lowering is caused by the delocalization of the impaired electron in the mixed-valence state.