Correlation and Negative Continuum Effects for the RelativisticM1Transition in Two-Electron Ions using the Multiconfiguration Dirac-Fock Method

The role of the E<-mc² continuum and of correlation in the multiconfiguration Dirac-Fock calculation of nondiagonal one-electron operators is described. Because of its importance and symmetry which emphasize both effects, we study the 1s2s³S₁→1s²¹S₀ relativistic $M1$ transition in two-electron ions. It is shown that both contributions have the same magnitude, and must be included even at low Z. These contributions improve significantly the agreement between theory and experiment at all Z, reducing the disagreement for Nb from $2.7$ to -1.8 times the experimental error.