The relativistic hydrogen atom: a theoretical laboratory for structure   functions
  Thanks to the Dirac equation, the hydrogen-like atom at high $Z$ offers a precise model of relativistic bound state, allowing to test properties of unpolarized and polarized structure functions analogous to the hadronic ones, in particular: Sivers effect, sum rules for the vector, axial, tensor charges and for the magnetic moment, positivity constraints, sea contributions and fracture functions 
X. Artru (IPNL), K. Benhizia