Complete Active Space Iterative Coupled Cluster Theory

In this work, we investigate the possibility of improving multireference-driven coupled cluster (CC) approaches with an algorithm that iteratively combines complete active space (CAS) calculations with tailored CC and externally corrected CC. This is accomplished by establishing a feedback loop between the CC and CAS parts of a calculation through similarity transformation of the Hamiltonian with those CC amplitudes that are not encompassed by the active space. We denote this approach the complete active space iterative coupled cluster (CASiCC) ansatz. We investigate its efficiency and accuracy in the singles and doubles approximation by studying the prototypical molecules H4, H8, H2O, and N2. Our results demonstrate that CASiCC systematically improves on the single-reference CCSD and the ecCCSD methods across entire potential energy curves, while retaining modest computational costs. However, the tailored coupled cluster method shows superior performance in the strong correlation regime suggesting that its accuracy is based on error compensation. We find that the iterative version of externally corrected and tailored coupled cluster methods converge to the same results.