Spin-free generalized normal ordered coupled cluster

Abstract

We present a spin-free, size-extensive, and numerically size-consistent coupled cluster method based on a generalized normal ordered exponential ansatz. This approach is a natural generalization of single-reference coupled cluster theory for arbitrary spin eigenfunctions. The working equations are size-extensive through the generalized normal order formalism and made spin-free with the spin-ensemble approach. Redundancies among excitations are eliminated by selecting only those excitations that project the reference function onto the first-order interacting space. Furthermore, by utilizing localized 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.