Spectroscopic constants of single-bond diatomic molecules and singlet-triplet gaps of diradicals by the block-correlated coupled cluster theory

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, CIF, and Cl-2) and the singlet-triplet gaps in a series of typical diradicals, including carbon, oxygen, and silicon atoms, methylene (CH2) and its isovalent species (NH2+, SiH2, and PH2+), 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.