A New Family of Seniority-Restricted Coupled Cluster Methods

We introduce a novel class of coupled cluster (CC) methods that leverage the seniority concept to enhance efficiency and accuracy in electronic structure calculations. While existing approaches, such as the pair coupled cluster doubles (pCCD) method, are limited to seniority-zero (Ω = 0) wave functions, we propose a more flexible framework: seniority-restricted coupled cluster (sr-CC). This new methodology selectively constrains the seniority sectors accessible through excitation operators in the cluster expansion, enabling a more systematic exploration of electron correlation effects. By balancing computational cost and accuracy, sr-CC provides a promising pathway for advancing electronic structure theory, particularly in strongly correlated systems. We benchmark sr-CCSD(0), sr-CCSDTQ(0), and sr-CCSDT(2)Q(0) on BeH₂ insertion, linear and cubic H₈, and F₂ dissociation. The sr-CC hierarchy remains robust in strongly correlated regimes where conventional single-reference CC methods deteriorate. For H₈ and F₂ dissociation, sr-CCSDT(2)Q(0) and sr-CCSDTQ(0) reproduce FCI energies to near machine precision across the full dissociation range, while bypassing the need for orbital optimization, although intermediate geometries remain more demanding. Overall, excitation-rank-dependent seniority restrictions provide a systematically improvable strategy for extending coupled cluster theory into strongly correlated regimes within a unified exponential framework.