Spontaneous symmetry breaking degeneracy

Theorem 13 Non-linear molecules in a spatially-degenerate electronic state are subject to spontaneous symmetry-breaking forces that distort the molecule to a geometry of lower symmetry, where the degeneracy is removed. 

SPONTANEOUS SYMMETRY BREAKING IN MATTER INDUCED BY DEGENERACIES AND PSEUDODEGENERACIES... 

Spontaneous Symmetry Breaking in Matter Induced BY Degeneracies and Pseudodegeneracies... 

Non-adiabatic coupling is also termed vibronic coupling as the resulting breakdown of the adiabatic picture is due to coupling between the nuclear and electi onic motion. A well-known special case of vibronic coupling is the Jahn-Teller effect [14,164-168], in which a symmetrical molecule in a doubly degenerate electronic state will spontaneously distort so as to break the symmetry and remove the degeneracy. 

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