  We performed accurate calculation of $\alpha$-dependence ($\alpha=e^2/hc$) of
the transition frequencies for ions, which are used in a search for the
variation of the fine structure constant $\alpha$ in space-time. We use
Dirac-Hartree-Fock method as a zero approximation and then the many-body
perturbation theory and configuration interaction methods to improve the
results. An important problem of level pseudocrossing (as functions of
$\alpha$) is considered. Near the crossing point the derivative of frequencies
over $\alpha$ varies strongly (including change of the sign). This makes it
very sensitive to the position of the crossing point. We proposed a
semiempirical solution of the problem which allows to obtain accurate results.
