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We apply degenerate many-body perturbation theory at high orders for theab-initio description of ground states and excitation spectra of open-shellnuclei using soft realistic nucleon-nucleon interactions. We derive a recursiveformulation of standard degenerate many-body perturbation theory that enablesus to evaluate order-by-order perturbative energy and state corrections up tothe 30th order. We study 6,7-Li as test cases using a similarityrenormalization group (SRG) evolved nucleon-nucleon interaction from chiraleffective field theory. The simple perturbation series exhibits a strong, oftenoscillatory divergence, as was observed previously for ground states ofclosed-shell nuclei. Even for very soft interactions resulting from SRGevolutions up to large flow parameter, i.e. low momentum scales, theperturbation series still diverges. However, a resummation of the perturbationseries via Pad\'e approximants yields very stable and converged ground andexcited-state energies in very good agreement with exact no-core shell-modelcalculations for the same model space.

Spectra of Open-Shell Nuclei with Pad\'e-Resummed Degenerate Perturbation Theory