Diagram approach to the calculation of the energy of a many-electron atom

The leading Feynman diagrams contributing to the energy of an atom are calculated. Numerical calculations of the energy matrices are carried out for the states of configurations 1s2s2 and 1s2p2 in three-electron atoms and 1s22s2 and 1s22p2 in four-electron atoms. For the case of Fe XXIV the theoretical wavelengths of the transition are compared with experimental data. The numerical calculation shows that the two-photon diagrams with two and three open ends make contributions of the same order of magnitude to the energy of the atom.