The two-time Green's Function and screened Self-Energy for two-electron   quasi-degenerate States
  Precise predictions of atomic energy levels require the use of QED, especially in highly-charged ions, where the inner electrons have relativistic velocities. We present an overview of the two-time Green's function method; this method allows one to calculate level shifts in two-electron highly-charged ions by including in principle all QED effects, for any set of states (degenerate, quasi-degenerate or isolated). 
E.-O. Le Bigot, P. Indelicato, V. M. Shabaev