School of Chemistry and Chemical Engineering, Key Laboratory of Mesoscopic Chemistry of the Ministry of Education, Institute of Theoretical and Computational Chemistry, Nanjing University, Nanjing 210093, People's Republic of China.
J Chem Phys. 2009 Nov 7;131(17):174101. doi: 10.1063/1.3256297.
Block correlated coupled cluster (BCCC) method with the complete active-space (CAS) self-consistent-field reference function (CAS-BCCC) has been applied to investigate the vertical excitation energies of low-lying valence excited states in a number of medium-sized organic molecules, including unsaturated aliphatic hydrocarbons (ethene, E-butadiene, cyclopropene, and cyclopentadiene), aromatic heterocycles (furan and pyrrole), and carbonyl compounds (formaldehyde, acetone, and formamide). An approximate CAS-BCCC with the cluster operator truncated up to the four-block correlation level, CAS-BCCC4, is employed in the calculations. The results are compared with those from the multireference configuration interaction with singles and doubles (MR-CISD and its corrected version, MR-CISD+Q), the complete active space with second-order perturbation theory (CASPT2), and CC3. Our results show that the overall performance of CAS-BCCC4 is competitive with that of the multistate CASPT2 (slightly inferior to MR-CISD+Q), better than that of the single-state CASPT2 and MR-CISD approaches. For triplet excited states, various methods tend to give relatively consistent predictions. However, for singlet excited states, various methods lead to quite different excitation energies in some cases.
采用完全活性空间自洽场（CAS）参考函数的关联耦合簇（BCCC）方法（CAS-BCCC），研究了一系列中等大小有机分子的低能价激发态的垂直激发能，包括不饱和脂肪烃（乙烯、E-丁二烯、环丙烯和环戊二烯）、芳香杂环（呋喃和吡咯）和羰基化合物（甲醛、丙酮和甲酰胺）。在计算中采用了截断到四联体相关水平的近似 CAS-BCCC（CAS-BCCC4）。将结果与多参考组态相互作用（MR-CISD 和其修正版本，MR-CISD+Q）、全活性空间二阶微扰理论（CASPT2）和 CC3 的结果进行了比较。结果表明，CAS-BCCC4 的整体性能与多态 CASPT2 相当（略逊于 MR-CISD+Q），优于单态 CASPT2 和 MR-CISD 方法。对于三重态激发态，各种方法往往给出相对一致的预测。然而，对于单重态激发态，在某些情况下，各种方法导致的激发能有很大差异。
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