B1-B2 phase transition of MgO by ab-initio molecular dynamics

Usually, thermal vibrations of atoms can be taken into account via the so-called quasiharmonic approximation (QHA). In this framework, the phonon dispersion relations are calculated at 0 K using density functional perturbation theory (DFPT) and the temperature is included only via the thermal dilatation, i.e., by computing the phonon spectrum at different volumes. The main drawback of this approximation is that its accuracy is difficult to assess since anharmonic contributions beyond quasiharmonicity are difficult to estimate. But if the harmonic part of the atomic vibrations dominate the free energy at elevated temperatures, the neglect or an inaccurate evaluation of seemingly minor contribution as the anharmonic one can result in falsely predicted phase stabilities or inaccurate phase transition temperatures.
Here we will present a recent method, called the temperature dependent energy potential (TDEP) that provides a consistent way to extract the best possible harmonic (or higher order) potential energy surface at finite temperatures. Then we will show some results on the phase transition between the B1 and B2 structures in MgO.