Full optimization of quasiharmonic free energy with an anharmonic lattice model: Application to thermal expansion and pyroelectricity of wurtzite GaN and ZnO

We present a theory and calculation scheme of structural optimization at finite temperatures within the quasiharmonic approximation (QHA). The is based on an efficient updating interatomic force constants with change crystal structures, which we call IFC renormalization. cell shape atomic coordinates are treated equally simultaneously optimized. apply to thermal expansion pyroelectricity wurtzite GaN ZnO, accurately reproduces experimentally observed behaviors. Furthermore, point out general obtain correct $T$ dependence lowest order in constrained optimizations that reduce number effective degrees freedom, helpful perform QHA calculations little sacrificing accuracy. show works properly for ZnO by comparing all freedom.