  The multiconfiguration Dirac-Hartree-Fock theory is used to calculate the 3d9
2D3/2 - 2D5/2 transition energy for Co-like ions with Z = 28-100. We
investigate how electron correlation, frequency-independent and -dependent
Breit interactions, as well as QED corrections, vary along the sequence. The
well-understood frequency-independent Breit contribution has the largest
contribution for all ions. Among the corrections to this, correlation decreases
rapidly with Z, the frequency-dependent Breit contribution is important
especially for high-Z ions, and the self-energy contribution to the QED becomes
the largest correction already for Z>50. We evaluate and compare results for
the Self-energy in three different approximations, (i) the approach implemented
in the GRASP2K package, (ii) the method based on Welton's concept and (iii) a
model operator approach recently developed by Shabaev and coworkers. Through
comparison with experimental values, it seems that the third set of results
have the best agreement with experiments, but the difference from experiments
for high-Z ions, is around 0.03%-0.04%, and therefore our results are outside
the error bars of the experiments.
