For the sake of better performance of MB-pol[1] and cheapness of the calculations, we made a supercell out of the ASE object before feeding it to the MB-pol calculator. It is done by passing three additional arguments to the MB-pol calculator named (X, Y, Z), which determines the number of replications in each cartesian direction.
It is worth noting that we also employed an energy-volume curve [2] and used the stress tensor to get more accurate energies. 

[1] https://github.com/paesanilab/mbpol_openmm_plugin
[2] https://phonopy.github.io/phonopy/qha.html#phonopy-qha