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Ab initio calculations with CI+MBPT of large number of energy levels for beryllium-like neon

Previously, ab initio precision configuration–interaction many-body perturbation theory (CI+MBPT) method was applied only to low energy levels. In this work, we consider highly excited states with this method and show that many theoretical levels agree well with experiment and can be easily identified. The precision of the method for lowest levels can be achieved at the level of tens of cm−1; however, for a large number of levels the precision is lower, a few 100 cm−1. This is due to slower convergence of bases obtained in large cavities to avoid cavity effects on the states of interest. Available experimental data have been used for label assignment and as a test of our method. This is the first attempt to apply ab initio precision method to hundreds of levels including moderately highly excited states. The method can be useful for level identification and for calculations of properties of Rydberg states in the regime where ion-core correlations are important and can be used to test other methods designed for these states.