  We calculated spectra of the first six members of the Mg-like isoelectronic
sequence using different approximations. The most accurate results were
obtained with the configuration interaction + all order method (CI+AO), which
provided relative accuracy for transition energies on the level of 0.1\%, or
better. On this level of accuracy the Breit and QED corrections become
important for the systems with nuclear charge $Z\gtrsim 20$. The retardation
part of the Breit interaction and QED corrections for the partial waves with
$l\ne 0$ are still too small to be important.
