The averaged dynamics of the hydrogen atom in crossed electric and   magnetic fields as a perturbed Kepler problem
  We treat the classical dynamics of the hydrogen atom in perpendicular electric and magnetic fields as a celestial mechanics problem. By expressing the Hamiltonian in appropriate action-angle variables, we separate the different time scales of the motion. The method of averaging then allows us to reduce the system to two degrees of freedom, and to classify the most important periodic orbits. 
Nils Berglund and Turgay Uzer