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

The role of the $E<\mathrm{}{-mc}^2$ 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 $1s2s{}^3{S}_1\to {1s}^2{}^1{S}_0$ relativistic $M1\mathrm{}$ 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 $-1.8\mathrm{}$ times the experimental error.