Projected Hartree-Fock theory as a polynomial of particle-hole excitations and its combination with variational coupled cluster theory
Henderson, Thomas M.
Scuseria, Gustavo E.
Projected Hartree-Fockﾠtheoryﾠprovides an accurate description of many kinds of strong correlations but does not properly describe weakly correlated systems.ﾠCoupled clusterﾠtheory,ﾠin contrast, does the opposite. It therefore seems natural to combine the two so as to describe both strong and weak correlations with high accuracy in a relatively black-box manner. Combining the two approaches, however, is made more difficult by the fact that the two techniques are formulated very differently. In earlier work, we showed how to write spin-projected Hartree-Fock in a coupled-cluster-like language. Here, we fill in the gaps in that earlier work. Further, we combine projected Hartree-Fock andﾠcoupled clusterﾠtheoryﾠin a variational formulation and show how the combination performs for the description of the Hubbard Hamiltonian and for several small molecular systems.