Abstract: The Fenske-Hall molecular orbital calculations were performed on the model cluster Cu_3O_(12)~(17-). The calculated results revealed that the valence levels are mainly composed of Cu d and O p components, and the levels near E_f are predominately of O p nature. Oxidation removes the antibonding electrons from the Cu (1) d_(z~2-y~2) MO and strengthens the chain Cu(1)—O(1) and Cu(1)—O(4) bonds so that they have slightly larger overlap population than the plane Cu(2)—O(2) and Cu(2)—O(3) bonds. The oxidation beyond the Cu~(2+) state does not lead to Cu~(3+) but to O~- state so that the O(1) as well as O(4) ions have slightly smaller electron charge than the O(2) and O(3) ions. The holes introduced by doping reside primarily on the Cu—O chain with dominantly sigma O(4) 2p_z character.