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.

Block Correlated Coupled Cluster Theory With A Complete Active-Space Self-Consistent-Field Reference Function: The General Formalism And Applications

“…For TMM, the calculated energy gaps for both singlet states (with respect to the ground triplet state) from four methods used here are comparable to each other, and the 3 A 2 − 1 A 1 gaps from all methods are quite consistent with the experimental value (16.1±0.1 kcal/mol) [178]. To conclude from the results presented here and in previous studies [128,130,131], one can find that the overall performance of CAS-BCCC4 and MR-CISD+Q methods are very competitive in predicting the singlet-triplet gaps of diradicals, significantly better than CASPT2 and MR-CISD. The performance of the CASPT2 method is even inferior to that of the MR-CISD method.…”

“…Fortunately, a computer program can be devised to achieve all these processes automatically. Some details for implementation have been discussed in our previous work [126][127][128]. It is worthwhile to mention the computational cost of CAS-BCCC4 calculations.…”

“…The CAS-BCCC4 method has been employed to study the singlet-triplet (S-T) gaps [128,130,131] for a number of typical diradicals including methylene (CH 2 ) and its isovalent species, substituted carbenes, benzyne isomer, and trimethylenemethane (TMM), etc. Here the adiabatic S-T gaps for methylene, halocarbenes (CF 2 , CCl 2 , and CBr 2 ), and TMM calculated with the CAS-BCCC4 method are listed in Table 6 1 and 1 B 1 ) are predicted to be of C 2v symmetry (not degenerate, due to the Jahn -Teller effect) [31,171,172].…”

“…For each molecule, the active space is chosen to be the minimum active space required for a qualitatively correct description of the broken bond. Some computational details were discussed previously [126,128]. The size-entensivity errors obtained with four theoretical methods are listed in Table 6-10.…”

“…Very recently, we have developed the general formalism of CAS-BCCC for generate active spaces [126,127], and reported its efficient implementation for groundstate and excited-state calculations [126][127][128][129][130][131][132][133]. In the CAS-BCCC method, a multiorbital block is defined to contain all active orbitals in the active space, and all other blocks involve just a single spin orbital (occupied or virtual).…”