We determine the equilibrium structures of CuO4 and CuO5 clusters within the framework of density functional theory and a plane-wave approach. Our calculations go beyond qualitative structural assignments proposed to interpret photoelectron spectroscopy measurements. We found that the lowest energy structures for CuO4 are based on Cu(O-2) units, isomers containing Cu(O-3) ozonide units being higher in energy. A different situation is observed in the case of CuO5, where several isomers with one O-3 motif lie at lower energy than those made of Cu(O-2) units only. Ozonide units appear crucial to ensure the stability of CuO5 clusters. This is achieved by a larger hybridization between Cu-3d-like and O-2p-like states, which persists when the ozonide unit transforms into an O-3 chain. (C) 2003 American Institute of Physics.