Open-Shell coupled-cluster method: Variational and nonvariational calculation of ionization potentials

A hermitian, variational open-shell coupled-cluster method is described and applied to the calculation of H₂O and N₂ ionization potentials in the _T ≈ T₂ approximation. A nonvariational calculation is also carried out, with the inclusion of T₁ and T₃ in addition to T₂. Both methods give fair agreement with experiment when only T₂ is taken into account. T₃, which is included at present in the nonvariational scheme only, has a considerable effect on the results and gives good agreement with experiment.