Full dynamic free energy minimisation is used to study the stability of different polymorphs of MgCl2. with a new set of interionic potentials derived from ab initio calculations using density functional theory. The calculated difference in free energies between the alpha- and beta-phases is extremely small: the vibrational contribution reverses the relative ordering of these two structures obtained in the static limit. We predict the thermal expansion of both forms is highly anisotropic and that, unusually, above approximate to 60 K the expansion is larger parallel than perpendicular to the layers.