  Accurate atomic structure calculations of complicated atoms with 4 or more
valence electrons begin to push the memory and time limits of supercomputers.
This paper presents a robust method of decreasing the size of \textit{ab
initio} configuration interaction and many-body perturbation theory
calculations without undermining the accuracy of the resulting atomic spectra.
Our method makes it possible to saturate the CI matrix in atoms with many
valence electrons. We test our method on the five-valence-electron atom
tantalum and verify the convergence of the calculated energies. We then apply
the method to calculate spectra and isotope shifts of tantalum's superheavy
analogue dubnium. Isotope-shift calculations can be used to predict the spectra
of superheavy isotopes which may be produced in astrophysical phenomena.
