Block-correlated Coupled Cluster Methods for Strongly Correlated Systems

I will report our recent advances in developing electronic structure methods for strongly correlated systems. With the generalized valence bond (GVB) wave function as the reference function, we have developed block-correlated coupled cluster method (GVB-BCCC in short) at the ab initio level for strongly correlated systems.1,2 The GVB-BCCC method in its present form is demonstrated to provide accurate descriptions for static correlation in strongly correlated systems. Our calculations on a number of typical systems have shown that GVB-BCCC4 (with up to four-pair correlation) can provide nearly exact ground-state energies as the density matrix renormalization group (DMRG) method for systems with large active spaces.3 An equation-of-motion GVB-BCCC method (EOM-GVB-BCCC) was developed to describe low-lying excited states for strongly correlated systems.4 The EOM-GVB-BCCC2b method with up to two-pair correlation has been implemented. Test calculations for a few strongly correlated systems have demonstarted that the S-T gaps from this method are quite consistent with the DMRG results. These results show that GVB-BCCC and EOM-GVB-BCCC are expected to be practical theoretical tools for ground-state and excited state calculations of strongly correlated systems with large active spaces.