QED calculations of intra-<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mi>L</mml:mi></mml:math>-shell singly excited states in Be-like ions

The ab initio approach is used to evaluate the excitation energies of the $2s2p{\phantom{\rule{0.16em}{0ex}}}^{2S+1}{P}_{J}$ states from the ground state as well as the $2s2p\phantom{\rule{0.16em}{0ex}}^{3}P_{1}\ensuremath{\rightarrow}2s2p\phantom{\rule{0.16em}{0ex}}^{3}P_{0}$ and $2s2p\phantom{\rule{0.16em}{0ex}}^{3}P_{2}\ensuremath{\rightarrow}2s2p\phantom{\rule{0.16em}{0ex}}^{3}P_{1}$ transition energies for selected Be-like highly charged ions over a wide range: from ${\mathrm{Ar}}^{14+}$ to ${\mathrm{U}}^{88+}$. The issue of a strong level mixing due to the proximity of states with the same symmetry is addressed by applying the QED perturbation theory for quasidegenerate levels. The employed approach combines a rigorous perturbative QED treatment up to the second order with electron-electron correlation contributions of the third and higher orders calculated in the Breit approximation. The higher-order QED effects are estimated using the model-QED-operator approach. The nuclear-recoil and nuclear-polarization effects are taken into account as well. The performed calculations are accompanied with a thorough analysis of uncertainties due to uncalculated effects. The most accurate theoretical predictions for the excitation and transition energies in Be-like ions are obtained, which, in general, are in perfect agreement with the available experimental data.