and hence the vibronic contribution is the only source of instability of any polyatomic system in nondegenerate states. If K 0 > 0 for the high-symmetry configuration of any polyatomic system, then a negative curvature, K = ( K 0 + K v ) < 0 , can be achieved only due to the negative vibronic coupling component K v , and only if | K v | > K 0 . It follows that any distortion of the high-symmetry configuration is due to, and only to the mixing of its ground state with excited electronic states by the distortive nuclear displacements realized via the vibronic coupling in Eq. (5). The latter softens the system with respect to certain nuclear displacements ( K v < 0 ), and if this softening is larger than the original (nonvibronic) hardness K 0 in this direction, the system becomes unstable with respect to the distortions under consideration, leading to its equilibrium geometry of lower symmetry, or to dissociation. There are many cases when neither the ground state is degenerate, nor is there a significant vibronic coupling to the lowest excited states to realize PJTE instability of the high-symmetry configuration of the system, and still there is a ground state equilibrium configuration with lower symmetry. In such cases the symmetry breaking is produced by a hidden PJTE (similar to a hidden JTE); it takes place due to a strong PJTE mixing of two excited states, one of which crosses the ground state to create a new (lower) minimum of the APES with a distorted configuration.