The C̃ 2A2 excited state of NO2: Evidence for a Cs equilibrium structure and a failure of some spin-restricted reference wavefunctions

Abstract

The geometry and harmonic vibrational frequencies of the the C̃ 2A2 electronic state of nitrogen dioxide have been determined using coupled cluster (CC) theory in conjunction with large basis sets. The minimum energy C2v-constrained N–O bond distance predicted by the highest level of theory is significantly shorter than the experimentally inferred value, and it is unlikely that further refinement in the theoretical treatment would reduce the discrepancy. However, this work suggests that a prevailing assumption of C2v symmetry may be incorrect; the equilibrium structure of the C̃ state may instead have Cs symmetry. In addition, and contrary to expectation, CC calculations based on variationally optimal spin-unrestricted or spin-restricted reference determinants give qualitatively different results for this system, unless very high levels of electron correlation are included in the calculation.