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 H 2 O and N 2 ionization potentials in the T ≈ T 2 approximation. A nonvariational calculation is also carried out, with the inclusion of T 1 and T 3 in addition to T 2 . Both methods give fair agreement with experiment when only T 2 is taken into account. T 3 , which is included at present in the nonvariational scheme only, has a considerable effect on the results and gives good agreement with experiment.