CALCULATIONS OF 5D(N-1)6S SYSTEMS USING ORTHOGONAL OPERATORS - DO ORTHOGONAL OPERATORS SURVIVE CONFIGURATION-INTERACTION

The merits and pitfalls of using orthogonal operators in systems with strong configuration interaction are illustrated by a study of a numbe r of (5d + 6s)(N) configurations (N = 8, 9) ranging from Ir to Pb. Bot h the perturbational and the full diagonalization approach are applied . Criteria are given to indicate the regions where either operators ba sed on perturbation theory or explicit configuration interactions are dominant. In both regions, orthogonal operators describe the (ground a nd) excited states well, provided they are extended by interconfigurat ion operators in the case of lower ionization, where full diagonalizat ion of the three (5d + 6s)(N) configurations is required. Ab initio ca lculations using second order perturbation theory show satisfactory ag reement with the results from fits to experimental energy levels, exce pt again for the lowest ionizations, where perturbation theory breaks down. It turns out to be a good approximation to exclude interactions treated by complete diagonalization from the perturbation expressions; both approaches can then be combined to describe excited state intera ctions.