  We use the combination of configuration interaction and many-body
perturbation theory method (CI+MBPT) to perform ab initio calculations the
low-energy spectra of Cr II with high accuracy. It is found that second-order
MBPT diagrams should be included in a consistent and complete way for the MBPT
to improve the accuracy of calculations in this five-valence-electron system.
This contrasts with previous ions with fewer valence electrons where it was
found that single-valence-electron diagrams dominate the corrections. Isotope
shifts and relativistic shifts (q-values) are calculated for use in
astronomical determination of the fine-structure constant in quasar absorption
spectra.
