  Applying the path integral formalism we study the equilibrium thermodynamics
of the phonon field both in the Holstein and in the Su-Schrieffer-Heeger
models. The anharmonic cumulant series, dependent on the peculiar source
currents of the {\it e-ph} models, have been computed versus temperature in the
case of a low energy oscillator. The cutoff in the series expansion has been
determined, in the low $T$ limit, using the constraint of the third law of
thermodynamics. In the Holstein model, the free energy derivatives do not show
any contribution ascribable to {\it e-ph} anharmonic effect. We find signatures
of large {\it e-ph} anharmonicities in the Su-Schrieffer-Heeger model mainly
visible in the temperature dependent peak displayed by the phonon heat
capacity.
