Lamb Shift in Light Hydrogen-Like Atoms
  Calculation of higher-order two-loop corrections is now a limiting factor in development of the bound state QED theory of the Lamb shift in the hydrogen atom and in precision determination of the Rydberg constant. Progress in the study of light hydrogen-like ions of helium and nitrogen can be helpful to investigate these uncalculated terms experimentally. To do that it is necessary to develop a theory of such ions. We present here a theoretical calculation for low energy levels of helium and nitrogen ions. 
Vladimir G. Ivanov (1), Savely G. Karshenboim (2 and 3) ((1) Pulkovo   Observatory, St. Petersburg, Russia (2) D. I. Mendele...