Corrections of Order $\alpha^6m$ to $S$-Levels of Helium
  Several new corrections of order $m\alpha^6$ to the energy levels of $S$ states in helium are obtained from radiative corrections to the Breit potential and from the polarization insertions in the two photon exchange graphs. While individual gauge invariant contributions are comparable to current experimental errors, the sum of these corrections is an order of magnitude smaller due to mutual cancellations. 
Michael I. Eides (Petersburg Nuclear Physics Institute, Russia, and   Pennsylvania State University, USA) and Howard Grotch ...