Problems in the calculation of intercombination line strengths

The calculation of weak intercombination line strengths sometimes requires an especially careful treatment of a combination of factors, including configuration mixing, spin-orbit interactions and relativistic effects. Fully relativistic ab initio MCDF calculations, when available, can provide an excellent guide to the Z-dependence of these transition probabilities for the upper portion of an isoelectronic sequence. For low to moderate ionization stages, good results may sometimes be obtained using an LS-coupled basis, MCHF or CI wavefunctions, and a treatment of the spin-mixing interactions which is guided by empirical data. These ideas are illustrated by examples from the configurations 2s3p in the Be isoelectronic sequence, 3s4p in the Mg sequence, and 3s3p3 in the Si sequence. Revised wavelength predictions for the 3s23p23P1,2-3s3p35S2 lines in the Si sequence are also presented. (orig.)