A unitary multiconfigurational coupled-cluster method: Theory and applications

A unitary wave operator exp(G) is used to relate a multiconfigurational reference function Φ to the full, potentially exact, electronic eigenfunction Ψ=exp(G)Φ. If the reference function Φ is of a generalized complete-active-space (CAS) form, then the energy, computed as 〈Φ‖exp(−G)H exp(G)‖Φ〉 is size extensive; here H is the full N-electron Hamiltonian. The Hausdorff expansion of exp(−G)H exp(G) is truncated at second order as part of our development. The parameters which appear in the cluster operator G are determined by making this second-order energy stationary. Applications to the widely studied H2O (at the double zeta basis level) and lowest and first excited 1A1 states of BeH2 are performed in order to test this method on problems where ‘‘exact’’ results are known.