Department of Chemistry, Virginia Tech, Blacksburg, Virginia 24061, USA.
J Chem Phys. 2010 Apr 14;132(14):144303. doi: 10.1063/1.3376073.
The ability of coupled-cluster models to predict vertical excitation energies is tested on the electronic states of carbon-chain radicals of particular relevance to interstellar chemistry. Using spin-unrestricted and -restricted reference wave functions, the coupled-cluster singles and doubles (CCSD) model and a triples-including model (CC3) are tested on the sigma radicals C(2)H and C(4)H. Both molecules exhibit low-lying excited states with significant double-excitation character (as well as states of quartet multiplicity) and are thus challenging cases for excited-state approaches. In addition, we employ two diagnostics for the reliability of the CC results: the approximate excitation level (AEL) relative to the ground state and the difference between excitation energies obtained with spin-unrestricted and spin-restricted reference wave functions (the U-R difference). We find that CCSD yields poor excitation energies for states with AEL significantly larger than ca. 1.1 and/or large U-R differences, as well as for certain states exhibiting large spin contamination or other inadequacies in the reference determinant. In such cases, connected triple excitations can be included in the model and generally provide improved results. Furthermore, we find that large discrepancies exist between CC and multireference (MR) results for certain states. These disagreements are not related to basis-set effects, but likely arise from the lack of spin adaptation in conventional spin-orbital CC implementations and active space selection in the MR models.
我们在对星际化学有重要意义的碳链自由基的电子态上测试了耦合簇模型预测垂直激发能的能力。使用无自旋限制和自旋限制参考波函数，我们测试了耦合簇单双激发（CCSD）模型和包含三重激发的模型（CC3）在西格玛自由基 C(2)H 和 C(4)H 上的表现。这两种分子都表现出具有显著双激发特征的低能激发态（以及四重态的态），因此是激发态方法的挑战性案例。此外，我们采用了两种诊断方法来评估 CC 结果的可靠性：相对于基态的近似激发能级（AEL）和无自旋限制和自旋限制参考波函数获得的激发能之间的差异（U-R 差异）。我们发现，对于 AEL 显著大于约 1.1 且/或 U-R 差异较大的态，以及对于某些表现出较大自旋污染或参考行列式其他不足的态，CCSD 会产生较差的激发能。在这种情况下，可以在模型中包含连接三重激发，通常会提供改进的结果。此外，我们发现对于某些态，CC 和多参考（MR）结果之间存在很大差异。这些差异与基组效应无关，但可能源于传统自旋轨道 CC 实现中缺乏自旋适应以及 MR 模型中活性空间选择。
