MBPT Calculations of Energy Levels for 1s2 and 1snl (N=2-5) Configuration of Helium Like Lithium Ion

In this paper, we carried out accurate energy levels calculations among the lowest 49 levels arising from the 1s2 and 1snl (n=2-5) configuration of the He-like-Li ion using the standard relativistic configuration interaction (RCI) approach and the second-order many body perturbation theory (MBPT). Both methods were implemented in the relativistic atomic code FAC. The self-consistent field approximation and the Hamiltonian effects of the Breit interaction as well as the QED effects were included to the different calculation methods. To assess the accuracy of our calculations, we performed comparison to available experimental (NIST database) and previous theoretical results. Comparisons are made with the available data in the literature and good agreement has been found which confirms the reliability of our results. Comparatively to the NIST database, it was found that our calculated energy levels using three methods, mainly standard FAC, RCI and MBPT are assessed to be mainly accurate to better than 1.33%, 0.47% and 0.06%. Our data are with great interest in plasma diagnostics.