The use of the second order perturbation correction, Eq. (4), for the calculation of the K v value in the case of PJTE instability is incorrect because in this case | K v | > K 0 , meaning the first perturbation correction is larger than the main term, and hence the criterion of applicability of the perturbation theory in its simplest form does not hold. In this case, we should consider the contribution of the lowest excited states (that make the total curvature negative) in a pseudo degenerate problem of perturbation theory. For the simplest case when only one excited state creates the main instability of the ground state, we can treat the problem via a pseudo degenerate two-level problem, including the contribution of the higher, weaker-influencing states as a second order correction. In the PJTE two-level problem we have two electronic states of the high-symmetry configuration, ground β β and excited γ γ , separated by an energy interval of 2 Δ Δ , that become mixed under nuclear displacements of certain symmetry Q = Q α α ; the denotations α α , β β , and γ γ indicate, respectively, the irreducible representations to which the symmetry coordinate and the two states belong. In essence, this is the original formulation of the PJTE. Assuming that the excited state is sufficiently close to the ground one, the vibronic coupling between them should be treated as a perturbation problem for two near-degenerate states. With both interacting states non-degenerate the vibronic coupling constant F in Eq. (5) (omitting indices) is non-zero for only one coordinate Q = Q α α with α α = β β γ γ . This gives us directly the symmetry of the direction of softening and possible distortion of the ground state. Assuming that the primary force constants K 0 in the two states are the same (for different K 0 see [1]), we get a 2×2 secular equation with the following solution for the energies ε ε ± ± of the two states interacting under the linear vibronic coupling (energy is referred to the middle of the 2 Δ Δ gap between the levels at the undistorted geometry):