Interplay between correlation and relativistic effects: 2s2 - 2s3p transitions in Be-like ions

Striking effects of relativity and correlation in the beryllium isoelectronic series are illustrated. For the transitions 2s2 - 2s2p 3P1 and 1P1, the singlet is higher in energy than the triplet and the intercombination line is relatively weak, which is the expected result. On the other hand, for the transitions 2s2 - 2s3p 3P1 and 1P1, there are values of Z in the isoelectronic series for which the transition strengths are of comparable magnitude, and values for which the triplet level is higher in energy than the singlet level. In order to make qualitatively correct predictions of these effects, a calculation must include correlation and relativity, as is the case for multiconfiguration Dirac-Fock methods