Precise Relativistic Configuration Interaction Calculation of the Atomic Structure of Helium-like Iron (Fe24+)

In this study, we present a relativistic configuration interaction (RCI) calculation of the atomic structure of helium-like iron (Fe24+, Z=26). The study examines the lowest 70 singly excited energy levels derived from the 1s nl configurations, where the principal quantum number n ≤ 6 and the orbital angular momentum l ≤ n–1. To evaluate the effects of configuration interaction and model convergence, several RCI models were developed by progressively expanding the configuration set. These include configurations up to 1s nl (n ≤ 8), as well as doubly excited states such as 2s²; 2s n′l′, 3s²; 3s n″l″ where n′ ≤ 6 and n″ ≤ 8. The calculated energy levels were compared with data available from the NIST Atomic Spectra Database. The results exhibit excellent agreement, with discrepancies generally within 0.1%, thereby validating the accuracy of our calculations. A detailed discussion on the convergence behavior of each model is also included to support the reliability of the method.