Ab Initio Anharmonic Free Energy from Phonon Quasiparticle: A Versatile and Easy-to-Use Script
Abstract
Knowledge of thermodynamics properties and phase relations of Earth forming phases under extreme conditions is critical to elucidate the nature Earth's and other planetary interiors. Ab initio simulations provide a stable and predictive theoretical framework to obtain these properties. In this regard, the quasi-harmonic approximation (QHA) has been widely employed despite the fact that sometimes this method is not appropriate. The QHA completely ignores intrinsic anharmonic effects, which, in general, become non-negligible at high temperatures approaching the melting temperature. To account for anharmonic effects, several ab initio approaches, such as many-body perturbation theory, self-consistent phonon, and molecular dynamics (MD) have been developed. Unfortunately, these approaches encounter difficulties when applied to complex crystal. To overcome these obstacles, we developed a hybrid approach which combines ab initio MD and lattice dynamics. Lattice vibrations containing infinite-order anharmonic effects are described in terms of phonon quasiparticle after a validity check of the existence of long lived normal modes. With this, it is possible to obtain anharmonic phonon dispersions over the whole Brillouin zone and free energies much more accurate than the one given by the QHA. The validity of this new approach to complex systems has been demonstrated in MgSiO3 perovskite and CaSiO3 perovskite. Recently, this method was also applied to beryllium. The predicted pre-melting hcp/bcc phase boundary compares well with experiments. A computational code now is available and the associated details are summarized in a recently submitted paper.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2018
- Bibcode:
- 2018AGUFMMR21B0057Z
- Keywords:
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- 3919 Equations of state;
- MINERAL PHYSICSDE: 3924 High-pressure behavior;
- MINERAL PHYSICSDE: 3954 X-ray;
- neutron;
- and electron spectroscopy and diffraction;
- MINERAL PHYSICSDE: 3994 Instruments and techniques;
- MINERAL PHYSICS