Intershell correlation corrections to the energy levels of the n=2 states of Li-like to F-like ions

The leading intershell correlation corrections to the energy levels of the 2sm2pk (m0-2, k=0-6) configurations of three- to nine-electron ions are calculated on the basis of the Z-expansion theory. These intershell correlations are leading corrections to the multiconfiguration Dirac-Fock (MCDF) results of Cheng and co-workers (1979) and the present study shows that they substantially improve the agreement between the MCDF results and experimental measurements.