Theory of Meyer-Neldel compensation for adiabatic charge transfer

At sufficiently high temperatures multi-phonon-assisted hopping of localized electronic charge carriers becomes semiclassical. Charge transfer then occurs when atoms' classical movements bring the electronic energy levels for a carrier at initial and final sites into temporary coincidence. Without rapid relaxation of a coincidence, dissipating vibrational energy away from the involved sites, an adiabatic hop is likely to be followed by a return hop. The lowering of the vibrational frequencies associated with a carrier's back-and-forth motion between sites augments the vibrational entropy of the hop's activated state. The proportionality of the additional vibrational entropy to the hop's activation energy, the minimum energy needed to produce a coincidence, establishes the Meyer-Neldel compensation effect.