Spin-flip configuration interaction: an electronic structure model that is both variational and size-consistent

A new formulation of the configuration interaction (CI) method is presented. It is based on the recently introduced spin-flip (SF) approach. SF-CI target states are described as spin-flipping excitations from the reference Hartree–Fock high-spin, e.g., Ms=1 (|αα〉), determinant. The resulting model is both variational and size-consistent. Moreover, the SF-CI model can describe within a single-reference formalism some inherently multi-reference situations, such as single bond-breaking and diradicals. Initial benchmarks for the SF-CI model with single and double substitutions (SF-CISD) are presented.