Error estimates for ab initio predictions for point defects in the Fe-N system

Density-functional theory has become one of the most prevalent methods to determine the structure and stability of crystalline solids. To obtain temperature-dependent free energies, however, which are essential in real-world materials science, the excitation mechanisms have to be taken explicitly into account. The quasi-harmonic approximation (QHA), where phonon spectra are calculated at different volumes to account for thermal expansion or contraction, has become a very interesting approximation as a trade-off between accuracy and computational resources. The aim for this project is to assess the accuracy of the QHA for the iron-nitrides Fe4N and Fe16N2, taking into account the effect of point defects in both phases.