Adiabatic approximation and the condition for the existence of a small polaron
Aleksandrov A. S.., Krebs A. B.
Perturbation theory, variational estimates, and numerical simulation by the Monte Carlo method have shown that adiabatic description of the interacting electrons and phonons based on the small ratio of the characteristic energy of a phonon, ω, to the kinetic energy of an electron, EF, is impaired in the region of intermediate values of the coupling constant λ ~ 1.