Figure 14: The effect of electron correlation beyond twophoton exchange—CoulombBreit NVPA, CoulombBreit retardation with and without Coulomb crossings, and CoulombBreit virtual pairs, all WITH electron correlation, for the groundstate of heliumlike ions (from [12, 13]). For comparison the effect of pure retarded twophoton exchange without additional correlation (included in the secondorder BetheSalpeter equation) is also indicated [6, Figure 10.10]. 
