Dreuw Andreas, Hoffmann Marvin
Interdisciplinary Center for Scientific Computing, Ruprecht-Karls University, Heidelberg, Germany.
Front Chem. 2023 Jul 27;11:1239604. doi: 10.3389/fchem.2023.1239604. eCollection 2023.
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 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.
例如，七嗪这类在第一激发单重态和三重态之间具有反转单重态-三重态能隙（STG）的分子，最近已有报道，并且由于它们违反了著名的洪德规则而受到了广泛关注。无论在分子的理论描述中（基组或电子相关水平）如何改进，利用最先进的高水平方法，单重态-三重态能隙都会消失并从下方趋近于零。看起来，我们观察得越仔细，反转单重态-三重态能隙的现象似乎越趋于消失。
