1b) has an inverted gap of E(S 1 -T 1 ) = -14 meV at the EOM-CCSD/aug-cc-pVDZ level. We attributed the inverted gap to its D 2h -symmetry pentalene core, which ensures non-overlapping HOMO and LUMO. Unlike pentalene, which distorts into C 2h -symmetry to alleviate antiaromaticity, the highly symmetric structure of isopyrene is aromatically stabilized by the pattern of 5-and 7-membered rings. Isopyrene is an azulenoazulene (an azulene dimer), and a similar aromaticity pattern and symmetry are found in the heptalenic isomer azupyrene, which is shown in Figure alongside azulene, which has C 2v symmetry. However, they both have small positive singlet-triplet gaps. In addition to isopyrene, we identified the D 2h -symmetry non-alternant hydrocarbon bicalicene, which features an expanded pentalene core resulting in a higher-order Hund's rule violation. This occurs between S 1 and T 2 , which are of the same electron configuration, while another lower energy triplet state is T 1 .