is positive, i.e., K > 0 . 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 Q α α , then K < 0 in this direction. The general formula for the energy is E = ⟨ ⟨ ψ ψ 0 | H | ψ ψ 0 ⟩ ⟩ , where H is the Hamiltonian and ψ ψ 0 is the wavefunction of the nondegenerate ground state. Substituting E in Eq. (1), we get (omitting the α α index for simplicity)