**Jahn-Teller effect** Synonyms: The Jahn–Teller effect, The Jahn-Teller distortion, the Jahn-Teller theorem.

The Jahn-Teller (JT) effect deals with molecular distortions due to an electronically degenerate ground state.

The HT effect describes the geometrical distortion of non-linear molecules under certain situations. It can be proven using group theory, that non-linear degenerate molecules cannot be stable.

Any non-linear molecule with a degenerate electronic ground state will undergo a geometrical distortion that removes that degeneracy, because the distortion lowers the overall energy of the complex.

For nonlinear molecular entities in a geometry described by a symmetry point group possessing degenerate irreducible representations there always exists at least one nontotally symmetric vibration that makes the electronic states with orbital degeneracy unstable at this geometry.

__First-order Jahn-Teller effect__ The nuclei are displaced to new equilibrium positions of lower symmetry causing a splitting of the originally degenerate electronic

states.

__Renner−Teller eﬀect__ For linear molecules.

__Pseudo Jahn-Teller or second-order Jahn-Teller effect__ In the case of molecules with a nondegenerate ground electron state, but with a very low lying excited state, distortions of proper symmetry arise which mix ground and excited states, and thereby lower the ground state energy.

The closer the states in energy, the more effective is the mixing.

__Source:__ http://old.iupac.org/publications/pac/1999/71_10_pdf/7110mirkin_1919.pdf Jahn–Teller effect - Wikipedia

http://en.wikipedia.org/wiki/Jahn%E2%80%93Teller_effect