Jahn-Teller interactions, degenerate electronic molecular states

Multicentre vibronic interactions are found in a wide number of systems such as impurity Jahn-Teller centres, molecular clusters and Jahn-Teller crystals [1], In these compounds the localized electrons in orbitally degenerate states interact directly only with active distortions of the local environment of corresponding vibronic centres. The distortions of different centres interact via the common vibrational modes of the system, which thus mediate the indirect interaction between Jahn-Teller centres [2,3]. As a result the corresponding vibronic problem becomes rather involved, with many electronic states mixed by many vibrational modes. 

Interaction between molecular vibrations and the orbital motion of electrons allows excitation of non-totally symmetric molecular vibrations during an electronic transition (breakdown of the Borti-Oppenheimer approximation). As an extreme manifestation of such an interaction, it happens that degenerate electronic states in non-linear molecules become unstable toward distortion (Jahn-Teller theorem). In fact, distortion due to non-totally symmetric vibrations will result in a lower molecular symmetry, with the consequence of lifting the orbital degeneracy [3]. 

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