The electronic spectrum of the blue copper protein plastocyanin has been studied by ab initio multiconfigurational second-order perturbation theory (the CASPT2 method). The six lowest electronic transitions have been calculated and assigned with an error of less than 2000 cm(-1). The singly occupied orbital in the ground state is Cu 3d-S-Cys 3p pi antibonding with some N-His 2p sigma character. The bright blue color originates from an electron transfer to this orbital from the corresponding Cu 3d-S-Cys 3p pi bonding orbital. The influence of different ligand models on the spectrum has been thoroughly studied; Cu(imidazole)(2)(SCH3)(S(CH3)(2))(+) as a model of CuHis(2)CysMet is the smallest system that gives converged results. The spectrum is surprisingly sensitive to changes in the geometry, especially in the Cu-S bond distances; a 5 pm change in the Cu-S-Cys bond length may change the excitation energies by as much as 2000 cm(-1). The effect of the surrounding protein and solvent on the transition energies has been modeled by point charges and is found to be significant for some of the transitions (up to 2000 cm(-1)).