3.1 Model quality Fig. shows the agreement between energies from the models based on the pair mode concept and the energies calculated from DFT. For the internal energies from DFT an RMSE of 4.76 meV/f.u. is obtained. Correcting the DFT energies for vibrational entropy results in a lower spread in energies and a smaller RMSE of only 3.79 meV/f.u. at 300 K. The models coarse grain over ionic relaxations etc., and considering the simplicity and short range nature of the model, the agreement 4 a Phonon dispersion and vibrational density of states for the ground state monoclinic structure of TMCMCdCl 3 . b Vibrational free energies of a set of metastable structures relative to that of the ground state. The solid lines indicate the calculated free energies from the phonon dispersions, and diamonds show the value from the correction model based only on the internal energy and the equilibrium volume of the metastable phases given in Eq. 3.