The 16 valence electronic states of nitric oxide dimer (NO)2

Sixteen electronic states of nitric oxide dimer are investigated using various ab initio levels of theory and various orientations of the dimer. These are the states which arise from the mixing of the singly occupied pi(NO)(*) orbitals of the monomers, and include all eight states which directly correlate to the (II)-I-2 ground states of the monomers. Twelve of the sixteen states are significantly multiconfigurational in character, which cause incorrect state orderings at low levels of theory. At several plausible geometries, eight low-lying states are predicted (four singlets and four triplets) within a 1 eV span, hence corresponding to excitations in the infrared, while the other eight states (six singlets and two triplets) lie much higher in the far ultraviolet, and in the realm of numerous other electronic states. The results imply, but do not confirm, that the only potential minimum lying below the lowest dissociation asymptote is the cis-ONNO geometrical conformation of the (X) over tilde(1)A(1) ground state.