For a better understanding it is important to follow up on how the PJTE is related to intramolecular interactions. In other words, what is the physical driving force of the PJTE distortions (transformations) in terms of well-known electronic structure and bonding? The driving force of the PJTE is added (improved) covalence: the PJTE distortion takes place when it results in an energy gain due to greater covalent bonding between the atoms in the distorted configuration. Indeed, in the starting high-symmetry configuration the wavefunctions of the electronic states, ground and excited, are orthogonal by definition. When the structure is distorted, their orthogonality is violated, and a nonzero overlap between them occurs. If for two near-neighbor atoms the ground state wavefunction pertains (mainly) to one atom and the excited state wavefunction belongs (mainly) to the other, the orbital overlap resulting from the distortion adds covalency to the bond between them, so the distortion becomes energetically favorable (Fig. 2).