Singlet-triplet gaps in substituted carbenes predicted from block-correlated coupled cluster method

Abstract

The block correlated coupled cluster (BCCC) method, with the complete active-space self-consistentfield (CASSCF) reference function, has been applied to investigating the singlet-triplet gaps in several substituted carbenes including four halocarbenes (CHCl, CF 2 , CCl 2 , and CBr 2 ) and two hydroxycarbenes (CHOH and C(OH) 2 ). A comparison of our results with the experimental data and other theoretical estimates shows that the present approach can provide quantitative descriptions for all the studied carbenes. It is demonstrated that the CAS-BCCC method is a promising theoretical tool for calculating the electronic structures of diradicals.block correlated coupled cluster method, carbene, singlet-triplet gap, diradicals

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.…”

“…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].…”

Comparison of some multireference electronic structure methods in illustrative applications

“…This result suggests that the use of the active space (4,4) is necessary for accurate predictions of the S-T gaps in HCCN. By comparing the performance of the various methods for these two diradicals and other diradicals studied previously [19,21,23], we can conclude that the overall performance of CAS-BCCC4 is competitive with MR-CISD+Q methods in predicting the S-T gaps of diradicals, and somewhat better than that of CASPT2 and MR-CISD. The performance of the CASPT2 method is even inferior to that of the MR-CISD method.…”

“…Various multireference methods have been employed for investigating a number of diradicals [19,21,23] including methylene, substituted carbenes, and benzyne isomers. Here, we will provide the adiabatic S-T gaps for cyanocarbene (HCCN) and isocyanocarbene (HCNC).…”

“…The CAS-BCCC4 method has been shown to provide satisfactory descriptions for a variety of chemical problems, in which the multireference character is significant. For example, our method has been applied to compute the potential energy surfaces (PESs) of bond breaking processes, the singlet-triplet (S-T) gaps in diradicals, the activation barriers for some isomerization reactions, spectroscopic constants in diatomic molecules, and excitation energies for low-lying excited states [18][19][20][21][22][23][24][25][26].…”