The neutral, cationic and anionic NiO2 species are studied using the density-functional theory with the local spin density approximation and generalized gradient approximation methods. The most stable geometry of the NiO2 species is found to be a linear ONiO (D-infinityh) for the neutral and anion and to be a cyclic Ni(O-2) (C-2v) for the cation. The electronic structure analyses show the Ni-O bondings in the NiO2 species have both covalent and ionic character. The low-lying excited states for the ONiO and Ni(O-2) isomers are calculated and used to assign the features in the photoelectronic spectrum. Our results compare well with the available experimental results. (C) 2003 American Institute of Physics.