Evaluation of thermodynamic properties of solids by quasiharmonic lattice dynamics

Quasiharmonic lattice dynamics is a simulation technique complementary to M onte Carlo and molecular dynamics. Quantum effects are readily taken into a ccount, and high precision does not normally require long runs. Vibrational stability is a sensitive test of interatomic potentials, and details of th e vibrational motion reveal mechanisms for phase transitions or for thermal expansion. The major computational task is usually to find the equilibrium geometry at a given T, P; done, calculating free energy, heat capacity, th ermal expansion, etc., is rapid and accurate. For three-dimensional ionic c rystals and slabs, our code SHELL calculates analytically first derivatives of the free energy with respect to all strains, internal us well as extern al, this gives a full minimization of the free energy so efficient that lar ge unit cells can be used, allowing applications to defects and disordered systems. Various applications are discussed: MgF2, including the rutile/flu orite transition: negative thermal expansion in ZrW2O8; anisotropic expansi on of polyethylene at very low temperatures; surface free energies for MgO; defect energies and volumes in MgO; and a new method for obtaining free en ergies and phase diagrams of disordered solids and solid solutions, applied to MnO/MgO and CaO/MgO.