The equilibrium geometry of any polyatomic system in nondegenerate states is defined as corresponding to the point of the minimum of the adiabatic potential energy surface (APES), where its first derivatives are zero and the second derivatives are positive. If we denote the energy of the system as a function of normal displacements  as , at the minimum point of the APES (), the curvature  of  in direction ,
is positive, i.e., . Very often the geometry of the system at this point of equilibrium on the APES does not coincide with the highest possible (or even with any high) symmetry expected from general symmetry considerations. For instance, linear molecules are bent at equilibrium, planar molecules are puckered, octahedral complexes are elongated, or compressed, or tilted, cubic crystals are tetragonally polarized (or have several structural phases), etc. The PJTE is the general driving force of all these distortions if they occur in the nondegenerate electronic states of the high-symmetry (reference) geometry. If at the reference configuration the system is structurally unstable with respect to some nuclear displacements , then  in this direction. The general formula for the energy is , where  is the Hamiltonian and  is the wavefunction of the nondegenerate ground state. Substituting  in Eq. (1), we get (omitting the  index for simplicity)
where  are the wavefunctions of the excited states, and the  expression, obtained as a second order perturbation correction, is always negative, . Therefore, if , the  contribution is the only source of instability. The matrix elements in Eq. (4) are off-diagonal vibronic coupling constants,
These measure the mixing of the ground and excited states under the nuclear displacements , and therefore  is termed the vibronic contribution. Together with the  value and the energy gap between the mixing states,  are the main parameters of the PJTE (see below). 
In a series of papers beginning in 1980 (see references in ) it was proved that for any polyatomic system in the high-symmetry configuration
and hence the vibronic contribution is the only source of instability of any polyatomic system in nondegenerate states. If  for the high-symmetry configuration of any polyatomic system, then a negative curvature, , can be achieved only due to the negative vibronic coupling component , and only if . 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 (), and if this softening is larger than the original (nonvibronic) hardness  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.