Thermodynamics of the molecular polaron

Abstract

The thermodynamics of the molecular polaron model-a two-level system coupled to a harmonic oscillator-is studied. Close upper and lower bounds to the free energy are derived by means of Tchebycheff-Markov inequalities and are compared with the results of other authors. The 'nearly free' behaviour for high temperatures is confirmed. The self-trapping effects at low temperatures are weaker than predicted earlier. In contrast to the prediction of mean-field theory, the transition between the two temperature regimes is not a sharp phase transition but rather a smooth crossover.