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

Abstract

We have presented in this chapter the general formalism of block correlated coupled cluster method with a CASSCF reference function (CAS-BCCC in short) and a number of its applications for electronic structure calculations of molecules with multireference character. The CAS-BCCC method has the following features: (1) free of the intruder states; (2) invariant with respect to orbital rotations within separated orbital subspaces (occupied, active, and virtual); (3) cost-effective; (4) core-extensive, but not size-extensive with respect to the total number of electrons. With the cluster operator truncated up to the four-block correlation level, the approximate CAS-BCCC method is named as CAS-BCCC4. The CAS-BCCC4 method is applied to investigate a number of chemical problems such as bond breaking potential energy surfaces, singlet-triplet gaps of diradicals, reaction barriers, spectroscopic constants of diatomic molecules, and low-lying excited states. Comparisons between results from CAS-BCCC4 and those from FCI or other theoretical methods demonstrate that the CAS-BCCC4 approach provides very accurate descriptions for all problems under study. The overall performance of CAS-BCCC4 is illustrated to be better than that of CASPT2 and MR-CISD methods.