Multi-reference calculations of nitric oxide dimer

The nitric oxide dimer, (NO)2, has been known as an archetype with severe near-degeneracy because of the weak N–N bonding. We thus performed a series of multi-reference calculations of fourth-order coupled pair approximation (MRCPA4) and configuration interaction (MRCI). For the ground state, the molecular structure of cis form was optimized by these calculations. The MRCPA4 geometry was favorably compared with the recent experimental data, indicating the importance of higher excitations. Low-lying singlet excited states were also addressed. Through these calculations, the intrinsic MR character of this system was illustrated.