Cancellation Effects in Computed Atomic Transition Probabilities

— The common method of computing atomic transition probabilities involves the expansion of atomic wavefunctions in terms of single-configuration, LS-coupled basis functions. Calculated values are frequently rendered very unreliable by cancellation effects resulting from intermediate-coupling and configuration-interaction mixing of basis functions. Examples are discussed in the spectra of Ar I, AI L, and Si I. The general circumstances under which computed transition probabilities may be expected to be most reliable are reviewed. 1. Qualitative discussion. — Theoretically computed atomic transition probabilities are of a notoriously low general level of accuracy. Although the reasons for this are fairly well known, we wish here to review the subject, with particular emphasis on configurationmixing effects, and using certain transitions in the spectra of Ar I, Al I, and Si I as illustrations. Weighted transition probabilities or oscillator strengths for electric dipole transitions are computed in terms of the line strength S according to the equa-