The present manuscript deals with a deep mathematical analysis of the functions that should be used in the Clausius Clapeyron thermodynamic modelling, which is usually modelled linearly, this research emphasizes that the thermal models are nonlinear even taking into account the heat capacities as a constant. Specifically, the mathematical analysis of the thermal decaying models proposes what equations should be used depending on the type of enthalpy function. Consequently, the enthalpy functions depend on the model of the heat capacityproposed by Albert Einstein.The magnitude of the errors that would be committing by using linear regressions on experimental data were also estimated; as is the case for the vast majority of publications in this area.The study also shows how the models behave in wide temperature range; and how they are becoming apparent linear as the temperature range decreases.