Degenerate and nondegenerate states in Jahn-Teller problems

A general discussion of the role of degeneracy in the Jahn-Teller effect (JTE) theory is discussed. The latest developments of the theory in combination with electronic structure calculations show that the pseudo JTE, which is based on vibronic coupling between two or more nondegenerate electronic states, does not require the energy gap A between them to be small. Large vibronic effects are possible for any A value, and the coupling to excited states is the only source of instability of the ground state. This allows for a more general formulation of the theorem of vibronic coupling effects, which makes the theory applicable to any polyatomic system. For ground vibronic energy levels, recent achievements show that they may be nondegenerate too. Degeneracy is thus not an inalienable attribute to polyatomic systems with strong vibronic coupling effects.