  We demonstrate that $V^{N-M}$ approximation is a good starting point for the
configuration interaction calculations for many-electron atoms and ions. $N$ is
the total number of electrons in the neutral atom, $M$ is the number of valence
electrons. $V^{N-M}$ is the self-consistent Hartree Fock potential for a
closed-shell ion with all valence electrons removed. Using of the $V^{N-M}$
approximation considerably simplifies the many-body perturbation theory for the
core-valence correlations. It makes it easier to include higher-order
correlations which often significantly improves the accuracy of the
calculations. Calculations for krypton and barium and their positive ions are
presented for illustration.
