Valence-specific open-shell coupled-cluster approach using a common vacuum. An application to doublet electronic states

We propose a simple valence-specific formulation of open-shell coupled-cluster theory using a single-exponential Ansنtz for the wave operator. Formally resembling the well-known Fock space coupled-cluster approach, our method is easy to implement and free from spin contamination, but avoids the necessity to generate the cluster amplitudes hierarchically, starting from the zero-valence problem. Illustrative applications to the low-lying doublet electronic states of the F atom and the OH molecule are reported.