1 mix strongly (the coefficients of these two leading configurations are C 1 ≈0.9 and C 2 ≈0.3, according to the EOM-SF-CCSD (EOM-CCSD with spin-flip) calculations ), posing the aforementioned challenges with (especially single-reference) quantum-chemical methods. A second feature consequence of the symmetry and energetic proximity of these two orbitals is that the ozone cation (which is isoelectronic to the NO 2 radical) has closely lying 2 A 1 and 2 B 2 electronic states derived by ionization from the HOMO-1 and HOMO-2, respectively (see Fig. ). Like the associated states in NO 2 , these two states are plagued by orbital symmetry breaking , a problem that greatly complicates their quantum-chemical treatment. One of the many accomplishments of the Bartlett group has been their integral role in the development of equation-ofmotion coupled-cluster theory (EOM-CC, also known as linear response coupled-cluster theory ). These methods provide a very efficient and simple way to treat certain classes of electronic structure that are often termed "multireference problems" , and are ideally suited to studying reactive intermediates, radicals, diradicals, and electronically excited states. From a somewhat wider viewpoint, the existence of closely spaced electronic states always carries the potential for (possibly strong) vibronic coupling, a phenomenon that can play an important role in molecular dynamics and spectroscopy. Indeed, one of the great successes of EOM-CC methods has been in their ability -if and only if combined with vibronic coupling models -to enable high-quality simulations of complicated electronic spectra. Such work provides important insights into the nature of vibronic coupling in molecular systems, as has been exemplified by various application studies (for example, see Refs. 26 and 27). In the case of ozone cation, the two lowest electronic states separated by a small gap of ∼0.1 eV are coupled by the asymmetric stretch of b 2 symmetry (Fig. ).