A compact spin-free cluster expansion formalism for simple open-shell configurations

A compact spin-free coupled cluster (CC) approach is presented for simple open-shell configurations such as doublets or h–p excited singlets/triplets, using a new cluster Ansaz for the wave-operator. Unlike the extant methods, the CC equations for state energies are at most quartic in the cluster amplitudes, analogous to the closed-shell theory. It can also be utilized for energy differences relative to a closed-shell singlet. The Ansaz allows contractions between cluster operators via spectator orbitals, taking care of relaxation and correlation effects very efficiently. Applications to IP and doublet energies demonstrate its efficacy for outer, inner and core ionizations.