What is Pseudo Jahn–Teller effect?

The pseudo Jahn–Teller effect , occasionally also known as second-order JTE, is a direct extension of the Jahn–Teller effect where spontaneous symmetry breaking in polyatomic systems occurs even in nondegenerate electronic states under the influence of sufficiently low-lying excited states of appropriate symmetry. "The pseudo Jahn–Teller effect is the only source of instability and distortions of high-symmetry configurations of polyatomic systems in nondegenerate states, and it contributes significantly to the instability in degenerate states".