Figure 14: The effect of electron correlation beyond two-photon exchange—Coulomb-Breit NVPA, Coulomb-Breit retardation with and without Coulomb crossings, and Coulomb-Breit virtual pairs, all WITH electron correlation, for the ground-state of helium-like ions (from [12, 13]). For comparison the effect of pure retarded two-photon exchange without additional correlation (included in the second-order Bethe-Salpeter equation) is also indicated [6, Figure  10.10].