It is well-known that the inclusion of the Breit interaction, alongside correlations, is important for an accurate theoretical understanding of the properties of heavy ions. In this work we include the Breit interaction in all-order calculations of energy levels and fine-structure intervals of ions in the Cs and Fr isoelectronic sequences using a Feynman-diagram technique. This requires modifying the electron Green's function to account for the Breit interaction within the all-order correlation potential method, which sums a dominating series of perturbative effects exactly. We find that Breit corrections to the energies of moderately charged ions along these sequences are very large, particularly for the $f$ states. Incorporating the Breit interaction into the all-order correlation potential provides a significant additional contribution beyond including it at the second-order level alone, substantially improving the fine-structure intervals. Furthermore, since the static Breit effect is so large for these systems, we also account for the frequency-dependent Breit interaction at the Dirac-Fock level and find that it does not significantly modify calculated atomic properties.

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• Atomic & molecular structure
• Atomic spectra