Block-Correlated Coupled Cluster Theory with up to Five-Pair Correlation for Accurate Static Correlation of High-Spin...

We present the implementation of generalized valence bond-based block-correlated coupled cluster theory with up to five-pair correlation (GVB-BCCC5) for accurate electronic structure calculations of high-spin strongly correlated (SC) systems. Several computational techniques have been adopted to render GVB-BCCC5 calculations practical for SC systems. The GVB-BCCC5 method is then applied to investigate low-lying electronic states of several challenging SC systems with relatively large active spaces, including three π-conjugated diradicals, an aminyl tetraradical, a Fe(II)-porphyrin model system, and an iron-sulfur cluster, [Fe₂S₂(SCH₃)₄]^2-. For all systems studied, GVB-BCCC5 can predict the energy differences between different spin states with an accuracy comparable to the density matrix renormalization group (DMRG) method. This work demonstrates that GVB-BCCC5 is a promising tool for the accurate description of high-spin SC systems within the active spaces.