The energy levels of a system, such as an atom that contains an odd number of spin-1/2 particles (e.g. electrons), are at least double degenerate in the absence of an external magnetic field. This degeneracy, known as Kramers degeneracy, is a consequence of time reversal invariance. Kramers theorem was stated by the Dutch physicist Hendrick Anton Kramers (1894–1952) in 1930. Kramers degeneracy is removed by placing the system in an external magnetic field. Kramers theorem holds even in the presence of crystal fields and spin–orbit coupling.
Subjects: Chemistry — Physics.