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The strong mixing of close levels with two valence electrons in Be-like xenongreatly complicates ab initio QED calculations beyond the first-orderapproximation. Due to a strong interplay between the electron-electroncorrelation and QED effects, the standard single-level perturbative QEDapproach may fail, even if it takes into account the second-order screened QEDdiagrams. In the present Letter, the corresponding obstacles are overcome byworking out the QED perturbation theory for quasidegenerate states. Thecontributions of all the Feynman diagrams up to the second order are taken intoaccount. The many-electron QED effects are rigorously evaluated in theframework of the extended Furry picture to all orders in the nuclear-strengthparameter $\alpha Z$. The higher-order electron-correlation effects areconsidered within the Breit approximation. The nuclear recoil effect isaccounted for as well. The developed approach is applied to high-precision QEDcalculations of the ground and singly excited energy levels in Be-like xenon.The most accurate theoretical predictions for the binding and excitationenergies are obtained. These results deviate from the most precise experimentalvalue by $3\sigma$ but perfectly agree with a more recent measurement.

Ab initio calculations of energy levels in Be-like xenon: strong interference between electron-correlation and QED effects