Second-order Rayleigh–Schrödinger perturbation theory for the GRASP2018 package: Three-particle Feynman diagram contribution to valence–valence correlations

The method based on the second-order perturbation theory to identify the most important configuration state functions of various correlations is extended to include valence–valence correlations, which are described by the three-particle Feynman diagram. The extension presented in this work complements the core–valence, core, core–core and valence–valence correlations which were developed in a series of previous papers by G. Gaigalas, P. Rynkun, and L. Kitovienė. Whereas these valence–valence correlations are described by the three-particle Feynman diagram, additional developments to calculate the spin-angular parts of this diagram have been made to the program library librang of the Grasp. As an example of the application of the developed method, the atomic calculations of the energy structure for the Se III ion are presented. In the present work, this method was also used to select the most significant configuration state functions and to use this basis to solve the self-consistent field equations.