Many cooperative JT systems would be expected to be metals from band theory, as to produce them, a degenerate orbital has to be partially filled and the associated band would be metallic. However, under the perturbation of the symmetry-breaking distortion associated to the cooperative JTE, the degeneracies in the electronic structure are destroyed and the ground state of these systems is often found to be insulating (see e.g.). In many important cases like the parent compound for colossal magnetoresistance perovskites, LaMnO 3, an increase of temperature leads to disorder in the distortions which lowers the band splitting due to the cooperative JTE, thus triggering a metal–insulator transition.