Forbidden transitions in the ground-state configuration of low-Z phosphorus-like ions

Systematically enlarged multiconfiguration Dirac-Fock wave functions have b een applied in order to investigate the forbidden lines among the 3s(2)3p(3 ) ground-state levels for phosphorus-like ions at the neutral end of the se quence. The probabilities and ratios of these lines are important in the di agnostics of astrophysical plasmas. In a recent computation by Keenan et al ., these probabilities were found to differ by up to similar to 25% from ea rlier (partially semiempirical) computations. To analyze this discrepancy i n the contest of ab initio structure theory, we report on a series of large -scale computations for these magnetic dipole and electric quadrupole trans ition probabilities. Our computations do not confirm the deviations found b y Keenan et al. Detailed comparisons are made with previous calculations an d observed data on the separation of energy levels and transition probabili ties for the four ions P I, S II, Cl III, and Ar IV. Our study clearly demo nstrates the possibility of a systemntic approach, but also shows the effor t that is needed to obtain sufficiently accurate atomic data for systems wi th five (or more) valence electrons.