Quantum Statistics of Hydrogen in Strong Magnetic Fields
  By an extension of the Feynman-Kleinert variational approach, we calculate the temperature-dependent effective classical potential governing the quantum statistical properties of a hydrogen atom in a uniform magnetic field. In the zero-temperature limit, we obtain ground state energies which are accurate for all magnetic field strengths from weak to strong fields. 
Michael Bachmann, Hagen Kleinert, and Axel Pelster