Abstract
We propose a fully ab initio based integrated approach to determine the volume and temperature dependent free-energy surface of nonmagnetic crystalline solids up to the melting point. The approach is based on density-functional theory calculations with a controlled numerical accuracy of better than 1 meV/atom. It accounts for all relevant excitation mechanisms entering the free energy including electronic, quasiharmonic, anharmonic, and structural excitations such as vacancies. To achieve the desired accuracy of for the anharmonic free-energy contribution without losing the ability to perform these calculations on standard present-day computer platforms, we develop a numerically highly efficient technique: we propose a hierarchical scheme—called upsampled thermodynamic integration using Langevin dynamics—which allows for a significant reduction in the number of computationally expensive ab initio configurations compared to a standard molecular dynamics scheme. As for the vacancy contribution, concentration-dependent pressure effects had to be included to achieve the desired accuracy. Applying the integrated approach gives us direct access to the free-energy surface for aluminum and derived quantities such as the thermal expansion coefficient or the isobaric heat capacity and allows a direct comparison with experiment. A detailed analysis enables us to tackle the long-standing debate over which excitation mechanism (anharmonicity vs vacancies) is dominant close to the melting point.
8 More- Received 23 December 2008
DOI:https://doi.org/10.1103/PhysRevB.79.134106
©2009 American Physical Society
Viewpoint
Turn off the lab furnace and boot up the mainframe
Published 13 April 2009
An old problem in solid-state physics is the difficulty of theory to account accurately for the heat capacity of solids close to their melting points. Ab initio calculations that can now better reconcile theory with experiment are poised to make such accurate predictions about new materials, it may not even be necessary to grow them.
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