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

中文翻译：

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像氙一样的能级从头算：电子相关性和QED效应之间的强干扰

在类似Be的氙气中，紧密能级与两个价电子的强烈混合大大使从头算QED的计算超出了一阶近似。由于电子-电子相关性和QED效应之间有很强的相互作用，即使考虑了二阶筛选的QED图，标准的单级微扰QED方法也可能会失败。在本函中，通过拟定准生成态的QED扰动理论克服了相应的障碍。考虑了所有费曼图直到二阶的贡献。在扩展的Furry图片框架内，严格评估了多电子QED效应，使其达到了核强度参数中的所有阶数$\alpha ž$。在Breit近似中考虑了高阶电子相关效应。核反冲效应也被考虑在内。所开发的方法适用于Be类氙气中地面和单激发能级的高精度QED计算。获得关于结合能和激发能的最准确的理论预测。这些结果偏离了最精确的实验值$3\sigma$ 但完全同意最近的测量。