Spin-free Generalised Normal Ordered Coupled Cluster

Abstract: We present a spin-free, size-extensive, and size-consistent coupled cluster method based on a generalised normal ordered exponential ansatz. This approach is a natural generalisation of single-reference coupled cluster theory for arbitrary spin eigenfunctions. The working equations are size-extensive through the generalised normal order formalism, and made spin-free with the spin-ensemble approach. Redundancies amongst excitations are eliminated by selecting only those excitations that project the reference function onto the first-order interacting space. Furthermore, by utilising localised orbitals, the proposed method describes dissociation into open-shell fragments size-consistently. Numerical results on prototypical multireference systems at the singles and doubles level of theory are competitive with existing multireference approaches, yet with more compact working equations.