Correlation and negative continuum effects for the relativistic M1 transition in two-electron ions using the multiconfiguration Dirac-Fock method

The role of the E < -mc(2) continuum and of correlation in the multiconfiguration Dirac-Fock calculation of nondiagonal one-electron operators is describe;l. Because df its importance and symmetry which emphasize both effects, we study the 1s2s S-3(1) --> 1s(2) S-1(0) 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.