Institute of Theoretical Chemistry, Faculty of Chemistry, University of Vienna, Vienna, Austria.
Vienna Doctoral School in Physics, University of Vienna, Vienna, Austria.
J Comput Chem. 2023 May 30;44(14):1381-1394. doi: 10.1002/jcc.27093. Epub 2023 Feb 24.
We present a large set of vertical excitation calculations for the ortho-nitrobenzaldehyde (oNBA) molecule, which exhibits a very challenging excited-state electronic structure like other nitroaromatic compounds. The single-reference methods produce mostly consistent results up to about 5.5 eV. By contrast, the CAS second-order perturbation theory (CASPT2) results depend sensitively on the employed parameters. At the CAS self-consistent field level, the energies of the bright states are strongly overestimated while doubly excited states appear too low and mix with these states. This mixing hampers the CASPT2 step, leading to inconsistent results. Only by increasing the number of states in the state-averaging step to about 40-to cover all bright states embedded in the double excitations-and employing extended multistate CASPT2 could CASPT2 results consistent with experiment be obtained. We assign the four bands in the molecule's spectrum: The weakest band at 3.7 eV arises from the states, the second one at 4.4 eV from the ( ) state, the shoulder at 5.2 eV from the ( ) state, and the maximum at 5.7 eV from the ( ) states. We also highlight the importance of modern wave function analysis techniques in elucidating the absorption spectrum of challenging molecules.
我们呈现了一组针对邻硝基苯甲醛（oNBA）分子的垂直激发计算，该分子表现出与其他硝基芳香族化合物非常相似的具有挑战性的激发态电子结构。单参考方法在大约 5.5 eV 之前产生的结果大多是一致的。相比之下，CAS 二级微扰理论（CASPT2）的结果对所采用的参数非常敏感。在 CAS 自洽场水平下，明亮态的能量被严重高估，而双激发态出现得太低并与这些明亮态混合。这种混合阻碍了 CASPT2 步骤，导致结果不一致。只有通过将态平均步骤中的态数增加到大约 40 个，以覆盖嵌入双激发态中的所有明亮态，并采用扩展的多态 CASPT2，才能获得与实验一致的 CASPT2 结果。我们对分子光谱中的四个谱带进行了归属：在 3.7 eV 的最弱谱带来自 态，在 4.4 eV 的第二个谱带来自 （ ）态，在 5.2 eV 的肩峰来自 （ ）态，而在 5.7 eV 的最大值来自 （ ）态。我们还强调了现代波函数分析技术在阐明具有挑战性的分子吸收光谱方面的重要性。
