Pseudo-Jahn-Teller Effect-A Two-State Paradigm in Formation, Deformation, and Transformation of Molecular Systems and Solids

The theoretical background formulation is to include the terms of nuclear displacements in the Hamiltonian and to solve such a nonadiabatic problem exactly. This approach is impractical. Fortunately, important solutions can be obtained by solving first the problem of electronic structure with fixed nuclei in the adiabatic approximation and then including the interaction of the electronic states with the nuclear displacements taken as a perturbation, thus reaching out beyond the adiabatic approximation. The similarity between the JTE and PJTE ends with the statement that both effects may distort the system, producing several equivalent minima of the APES at which the system has lower symmetry. For a better understanding the chemical implications of the PJTE, it is important to follow up on how this effect is related to intramolecular interactions.