Jahn–Teller effect

The Jahn–Teller effect is a distortion of a nonlinear system with an electronically degenerate ground state that removes the degeneracy and lowers its energy.

Electronic degeneracy: A condition in which two or more distinct quantum states have the same energy. The effect begins when a nonlinear system has a degenerate electronic ground state.
Octahedral complex: A coordination complex with six ligands arranged around a central atom at the vertices of an octahedron. Octahedral coordination provides the common setting for metal-ion distortions.
Molecular symmetry: The symmetry operations that leave a molecule or structure unchanged. Symmetry determines which electronic degeneracies and distortive modes are possible.
Peierls transition: A structural distortion in a one-dimensional metal that opens an electronic gap and lowers its energy. It is a related instability driven by electronic energy lowering, but in a different setting.
Vibronic coupling: Interaction between electronic states and vibrational motion in a molecule or solid. It links electronic state energies to distortions of the nuclei.
Copper(II) complexes: Coordination compounds containing copper in the +2 oxidation state. Their d9 electronic configuration often produces pronounced octahedral distortions.
Point group: The set of symmetry operations that leave an object fixed at a point. Point-group symmetry classifies states and distortions in molecular examples.
Berry phase: A geometric phase acquired by a quantum state as system parameters vary around a closed path. Conical intersections associated with the effect can produce a geometric phase.
Potential energy surface: A representation of a system’s potential energy as a function of its nuclear coordinates. A degenerate surface can slope downward along distortive nuclear coordinates.
Orbital ordering: A spatially organized pattern of occupied orbitals in a solid. Local distortions can align across a crystal through ordered orbital occupancy.

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