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This paper introduces a novel coupled cluster (CC) approach for multireference and open-shell molecular systems by employing a restricted active space (RAS) framework. By restricting the cluster operator to excitations from doubly occupied to virtual orbitals within the RAS, the commutativity of individual operators is ensured, enabling efficient computation. The method, demonstrated with RAS-CCSD, accurately captures dynamical correlation for challenging electronic structures through diagonalization of the similarity-transformed Hamiltonian.
A simplified coupled cluster method unlocks accurate treatment of multireference and open-shell molecules by leveraging restricted active spaces and similarity transformations.
We introduce a simple coupled cluster approach for multireference and open-shell systems. In the restricted active space coupled cluster framework, the cluster operator only includes excitations from doubly occupied to virtual orbitals, and this ensures the individual operators commute among each other. The ground and excited states are obtained by diagonalisation of the coupled cluster similarity transformed Hamiltonian in a restricted active space. We demonstrate the framework for the RAS-CCSD model, where single and double excitations are included in the cluster operator, and likewise for a complete active space. We consider systems with multireference character and open-shell systems in a spin-adapted manner. The approach captures significantly more dynamical correlation than without the coupled cluster similarity transformation.