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

Abstract

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.

New Approaches for ab initio Calculations of Molecules with Strong Electron Correlation

“…This observation was already made for CASPT2 in Ref. 121. In addition, we also observe a similar trend for DMRG-SCF/NEVPT2.…”

Describing Strong Correlation with Block-Correlated Coupled Cluster Theory

“…These operators have been defined and discussed in the CAS-BCCC method we proposed previously. 30,31 The CAS(2,2)-BCCC method (identical to GVB(1)-BCCC) has been demonstrated to provide a quantitative description for the single bond breaking process and the ground-state electronic structure of many diradicals. Thus, we can predict that with inclusion of geminal-virtual and core-virtual cluster operators, GVB-BCCC can provide a balanced description of static (or intergeminal) and dynamic electron correlation.…”

State‐specific multireference coupled‐cluster theory

“…Yet another alternative internally contracted scheme is block correlated CC theory (BCCC) 128,129. Instead of representing the determinants of the model space by the occupancies of the active orbitals, in BCCC theory the model space is regarded as one ‘block’ and sliced into the set of physical states for all possible numbers of active electrons.…”

“…As a result, BCCC lacks full size extensivity. Since the size extensivity errors steeply increase with the size of the active space,129 one potential alternative would be to use smaller active blocks consisting only of orbital pairs by switching to a generalized valence bond reference function.…”