AUTHOR=Dreuw Andreas , Hoffmann Marvin TITLE=The inverted singlet–triplet gap: a vanishing myth? JOURNAL=Frontiers in Chemistry VOLUME=11 YEAR=2023 URL=https://www.frontiersin.org/journals/chemistry/articles/10.3389/fchem.2023.1239604 DOI=10.3389/fchem.2023.1239604 ISSN=2296-2646 ABSTRACT=

Molecules with an inverted singlet–triplet gap (STG) between the first excited singlet and triplet states, for example, heptazine, have recently been reported and gained substantial attention since they violate the famous Hund’s rule. Utilizing state-of-the-art high-level ab initio methods, the singlet–triplet gap vanishes and approaches zero from below whatever is improved in the theoretical description of the molecules: the basis set or the level of electron correlation. Seemingly, the phenomenon of inverted singlet–triplet gaps tends to vanish the closer we observe.