title: Block correlated coupled cluster method with the complete active-space self-consistent-field reference function: Applications for low-lying electronic excited states.
abstract: 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.
authors.affiliation: 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.
authors.forename: Jun
authors.initials: J
authors.lastname: Shen
authors.forename: Shuhua
authors.initials: S
authors.lastname: Li