In this work we have carried out ab initio electronic structure calculations, CASSCF/CASPT2 and CASSCF/MRCI-SD+Q with several Pople＇s and correlation-consistent Dunning＇s basis sets, of the planar cis- and trans-NO dimers for the lowest eight electronic (singlet and triplet) states. The geometry, frequencies, dipole moment, binding energy, and vertical excitation energies are predicted with an accuracy close to or even better than the best reported ab initio previous results for some of these properties, and in very good agreement with the available experimental data. CASPT2 optimized geometries show the existence of at least four shallow NO-dimers (i.e., two cis-(NO)(2) ((1)A(1) and B-3(2)) and two trans-(NO)(2) ((1)A(g) and (3)A(u))), although CASSCF optimization with CASPT2 pointwise calculations indicate the existence of other less stable dimers, on the excited states. Vertical excitation energies were calculated for these four dimers. For the cis-NO dimer, the ordering and the energy spacings between the excited states (i.e., (1)A(1), B-3(2), B-1(2), 2nd (1)A(1), (1)A(2), (3)A(2), B-3(1), 2nd B-3(1)) are very similar to those found in a recent MRCI-SD study. The singlet cis-NO dimer ((1)A(1)) is the most stable one in almost quantitative accord with the experimental data, and in disagreement with previous density functional theory studies. A nonplanar transition state for the singlet trans <----> cis isomerization has also been fully characterized. This leads to an almost negligible energy barrier which would originate a rapid isomerization to the most stable cis-NO dimer at low temperatures, in accord with the experimental difficulties to measure the properties of the trans-NO dimer. Not only are basis set superposition error corrections necessary to evaluate accurately the binding energies, but also to determine the NN distance of these symmetrical dimers. Some problems regarding the symmetry of the wave function were found for the symmetrical NO dimers and for the NO+NO asymptote, and several approximate solutions were proposed. (C) 2000 American Institute of Physics. [S0021-9606(00)31115-1].