Hidden PJTE, spin crossover, and magnetic-dielectric bistability. As mentioned above, there are molecular systems in which the ground state in the high-symmetry configuration is neither degenerate to trigger the JTE, nor does it interact with the low-lying excited states to produce the PJTE (e.g., because of their different spin multiplicity). In these situations, the instability is produced by a strong PJTE in the excited states; this is termed "hidden PJTE" in the sense that its origin is not seen explicitly as a PJTE in the ground state. An interesting typical situation of hidden PJTE emerges in molecular and solid-state systems with valence half-filed closed shells electronic configurations e and t. For instance, in the e case the ground state in the high-symmetry equilibrium geometry is an orbital non-degenerate triplet A, while the nearby low-lying two excited electronic states are close-in-energy singlets E and A; due to the strong PJT interaction between the latter, the lower component of E crosses the triplet state to produce a global minimum with lower symmetry. Fig. 3 illustrates the hidden PJTE in the CuF 3 molecule, showing also the singlet-triplet spin crossover and the resulting two coexisting configurations of the molecule: high-symmetry (undistorted) spin-triplet state with a nonzero magnetic moment, and a lower in energy dipolar-distorted singlet state with zero magnetic moment. Such magnetic-dielectric bistability is inherent to a whole class of molecular systems and solids.