Calculated vertical excitation energies, optimized geometries, and vibrational frequencies of the nitric oxide dimer are reported. The "multireference'' (MR) nature of the problem and weak bond between the monomers make a proper description of the system difficult, and standard methods are not as applicable to this system. In this study, recently developed methods such as the double-electron-affinity similarity-transformed equation-of-motion coupled cluster method (DEA-STEOM-CCSD), MR Brillouin-Wigner CCSD(MR-BWCCSD), MR average quadratic CCSD (MR-AQCCSD), and others are used along with a series of basis sets of increasing accuracy. The calculated excitation energies are consistent and convergent with respect to the basis set in DEA-STEOM-CCSD, MR-BWCCSD, and MR-AQCCSD methods. The geometries are highly sensitive to the basis set size and the challenge to obtain the right answers in the basis set limit remains. Nevertheless, we obtain qualitative agreement with the experimental geometry and harmonic vibrational frequencies. The results from the above multireference methods show dramatic improvement over the coupled cluster with singles and doubles and perturbative triples excitation [CCSD(T)] results. Like O-3, (NO)(2) offers an extremely challenging example in its ground and excited states for single-reference and multireference theory. It deserves to be a standard test molecule as new methods are developed. (C)2003 American Institute of Physics.