I want to show which mechanism (bond proportion effect, volume effect or size mismatch effect) that lead to significant entropy contribution to the free energy. Figure displays the volumes of the configurations (green points) that are used to fit the cluster expansion. They show a completely positive deviation from the Vegard law (I have no idea if it is correct here) and the lattice constants of the two end configurations are 2.85 and 3.15 angstrom, respectively. Thanks.
I am assuming that you are computing vibrational contribution too here (not just an energy cluster expansion).
You could try to recompute the vibrational free energies
after artificially changing the volume of the structures to obey Vegard’s law.
after artificially placing the atoms on their ideal lattice sites (undoing relaxations).
both of the above.
This would tell you if volume or relaxation effects are important.