Kramer's theorem

The Kramers degeneracy theorem states that the energy levels of systems with an odd total number of electrons, protons, and neutrons (i.e. the total number of fermions) remain at least doubly degenerate in the presence of purely electric fields (i.e. no magnetic fields).

Examples

H has one electron and one proton. Not an odd number of fermions. Kramer's theorem does not apply.

D has one electron, a proton and a neutron. An odd number of fermions. Kramer's theorem applies.

__Source:__ http://en.wikipedia.org/wiki/Kramers_theorem