Institute of Chemistry , Eötvös Loránd University , H-1117 , Budapest , Hungary.
Quantum Theory Project, Department of Chemistry and Physics , University of Florida , Gainesville , Florida 32611 , United States.
J Chem Theory Comput. 2018 Nov 13;14(11):5859-5869. doi: 10.1021/acs.jctc.8b00681. Epub 2018 Oct 23.
The validation of the quality of the description of excited electronic states is of special importance in quantum chemistry as the general reliability of ab initio methods shows a much larger variation for these states than for the ground state. In this study, we investigate the quality of excited state energy gradients and potential energy surfaces on selected systems, as provided by the single reference coupled cluster variants CC2, CCSD, CCSD(T)(a), and CC3. Gradients and surface plots that follow the Franck-Condon forces are compared to the respective CCSDT reference values, thereby establishing a useful strategy for judging each variant's accuracy. The results reveal serious flaws of lower order methods - in particular, CC2 - in several situations where they otherwise give accurate vertical excitation energies, as well as excellent accuracy and consistency of the recently proposed CCSD(T)(a) method.
激发电子态描述质量的验证在量子化学中尤为重要，因为从头算方法的总体可靠性对于这些态的变化比对于基态的变化要大得多。在这项研究中，我们研究了由单参考耦合簇变体 CC2、CCSD、CCSD(T)(a)*和 CC3 提供的选定体系的激发态能量梯度和势能面的质量。遵循 Franck-Condon 力的梯度和表面图与各自的 CCSDT 参考值进行了比较，从而为判断每个变体的准确性建立了一种有用的策略。结果表明，在某些情况下，较低阶方法（特别是 CC2）存在严重缺陷，尽管它们在其他情况下给出了准确的垂直激发能，以及最近提出的 CCSD(T)(a)*方法的出色准确性和一致性。
