Effect due to the odd terms in the vibronic perturbation expansion. In the case of molecules with a non-degenerate ground electronic state but with a low-lying degenerate excited state, distortions of proper symmetry arise that result in mixing of the ground and excited states, thereby lowering the ground-state energy (pseudo Jahn–Teller effect). The pseudo Jahn–Teller effect manifests itself in fluxional behaviour (see fluxional molecules) and stereochemical non-rigidity of molecules and ions. The Jahn–Teller effect generates a surface crossing (e.g., a conical intersection) whereas a pseudo Jahn–Teller effect generates an avoided crossing.