Abstract
Taking as a benchmark the accurate calculations on 1,3S, 1,3P, and 1,3D resonances under the N=2 threshold, carried out for two-electron systems, we present an appraisal of the practical aspects of, and equivalence between, existing schemes to classify doubly excited states. To relate properties of resonances for different values of the nuclear charge Z, we have also carried out calculations of their positions and widths for noninteger Z and have drawn the ensuing correlation diagrams. We show that however good the descriptions are they do not provide a completely general scheme to classify resonances. Furthermore, the proposed schemes are not equivalent, as is usually assumed.