title: Spectroscopic constants of single-bond diatomic molecules and singlet-triplet gaps of diradicals by the block-correlated coupled cluster theory.
abstract: The spatial orbital formulations of block-correlated coupled cluster (BCCC) theory with a complete active-space self-consistent-field (CASSCF) reference function and its efficient implementation is presented. In the present implementation, the cluster operator is truncated to the four-block correlation level, and the CASSCF(2,2) reference function is assumed (thus, the method is abbreviated as CAS-BCCC4). We have applied this method to investigate the spectroscopic constants in seven single-bond diatomic molecules (LiH, HF, HCl, Li(2), F(2), ClF, and Cl(2)) and the singlet-triplet gaps in a series of typical diradicals, including carbon, oxygen, and silicon atoms, methylene (CH(2)) and its isovalent species (NH(2)(+), SiH(2), and PH(2)(+)), and three benzyne isomers. A comparison of our results with the experimental data or other theoretical estimates shows that the present approach can provide quantitative descriptions for all of the studied systems.
authors.affiliation: School of Chemistry and Chemical Engineering, Key Laboratory of Mesoscopic Chemistry of 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: Tao
authors.initials: T
authors.lastname: Fang
authors.forename: Weijie
authors.initials: W
authors.lastname: Hua
authors.forename: Shuhua
authors.initials: S
authors.lastname: Li