PJTE versus Renner–Teller effect in bending distortions. Linear molecules are exceptions from the JTE, and for a long time it was assumed that their bending distortions in degenerate states (observed in many molecules) is produced by the Renner–Teller effect (RTE) (the splitting of the generate state by the quadratic terms of the vibronic coupling). However, recently it was proved that the RTE, by splitting the degenerate electronic state, just softens the lower branch of the APES, but this lowering of the energy is not enough to overcome the rigidity of the linear configuration and to produce bending distortions. It follows that the bending distortion of linear molecular systems is due to, and only to the PJTE that mixes the electronic state under consideration with higher in energy (excited) states. This statement is enhanced by the fact that many linear molecules in nondegenerate states (and hence with no RTE) are, too, bent in the equilibrium configuration. The physical reason for the difference between the PJTE and the RTE in influencing the degenerate term is that while in the former case the vibronic coupling with the excited state produces additional covalent bonding that makes the distorted configuration preferable (see above, section 2.3), the RTE has no such influence; the splitting of the degenerate term in the RTE takes place just because the charge distribution in the two states becomes nonequivalent under the bending distortion.