Second order of Rayleigh–Schrödinger perturbation theory for the Grasp2018 package: Core correlations

Abstract

The paper presents a further development of the method presented in G. Gaigalas, P. Rynkun, and L. Kitovienė, Second-order Rayleigh–Schrödinger perturbation theory for the GRASP2018 package: core–valence correlations, Lith. J. Phys. 64(1), 20–39 (2024) (https://doi.org/10.3952/physics.2024.64.1.3), based on a combination of the relativistic configuration interaction method and on the stationary second-order Rayleigh–Schrödinger many-body perturbation theory in an irreducible tensorial form. In this extension, the perturbation theory accounts for both electron core–valence and core correlations when an atom or ion has any number of valence electrons, while the relativistic configuration interaction accounts for the rest of correlations. This allows a significant reduction of the space of the configuration state functions for complex atoms and ions. We also demonstrate how this method works for the energy structure calculation of Fe XV ion.