[Wien] Energy differences - Transition temperature

[Michael Fischer]

Hello everyone,

I am facing a problem, caused by a lack of knowledge in the field of thermodynamics:
I have a system that undergoes several phase transitions. From experiment, I know the phase transition temperatures as well as the excess entropies, which were obtained from heat capacity measurements by subtracting the Debye lattice contribution and calculating the integral below the curve of the Cp anomaly.
I have calculated the total energy for phases of different using the WIEN2k package. From x-ray diffraction data, I am quite confident that these phases represent the real symmetry of the different phases, and I can qualitatively relate them to the transition temperatures.
(Sequence cubic-orthorhombic-monoclinic on cooling)
Now, I am looking for a quantitative relationship between the differences in energy obtained from the ab-initio calculations for the different phases to the transition temperatures/excess entropies. It is clear that this relationship could only be an estimation, as temperature effects are neglected in the calculation.
Any help concerning this problem, particularly references covering this topic, would be highly appreciated.

Thank you very much, Greetings
Michael Fischer
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