Electronic structure of the Cvetanović diradicals is complex, featuring a manifold of closely lying electronic states derived by distributing 4 electrons over 3 orbitals. To simplify the description of electronic states, in the case of ethylene adduct we considered a symmetric geometry of the lowest triplet state (with the CCO symmetry plane), as was done in previous studies (the structures and relevant energetics is shown in Fig. in the SI). Figure shows frontier MOs of C 2 H 4 O at the optimized geometry of the 3 A state and the leading electronic configurations of the relevant states. Although the exact shape of orbitals depends on the method, their characters can be reliably described as out-of-plane lone pair on oxygen, lp(O), and bonding-antibonding pair formed by in-plane p-orbital of oxygen and p-orbital of carbon. Different electronic distributions of 4 electrons on these 3 orbitals give rise to several closely lying electronic states. The leading electronic configurations of the relevant electronic states are shown in the middle panel of Fig. . At symmetric (C s ) structures, one can generate 2 different triplet states ( 3 A and 3 A ) and two open-shell singlets with the same orbital occupations. At low-symmetry structures, all electronic configurations of the same multiplicity can mix, giving rise to highly multi-configurational wavefunctions. This multi-configurational character and multiple closely lying electronic states pose a challenge for theory.