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Title: Supercell electrostatics of charged defects in periodic density functional theory
Author: Durrant, Thomas Robert
ISNI:       0000 0004 8507 6812
Awarding Body: UCL (University College London)
Current Institution: University College London (University of London)
Date of Award: 2019
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Charged defects are often studied using density functional theory (DFT). However, the use of periodic boundary conditions (PBC) introduces a fictitious jellium background required to maintain charge neutrality, along with periodic images of the defect. As a consequence, the calculations are found to be sensitive to the size of the supercell used. Several competing correction methods have been developed for bulk materials, but do not provide a completely satisfactory solution to this problem. One source of error in these corrections is suggested to be the simple density models employed. I propose a new electrostatic correction based on calculating electronic charge density differences between reference DFT calculations, and then solving Poisson equation for these charge models. For bulk materials, I demonstrate that this new correction is in agreement with the existing Lany-Zunger (LZ) method. This demonstrates that using more realistic density models is not sufficient to improve the corrections, but the new method does allow an improved understanding of potential alignment. I found that alignment errors are introduced by adding or removing atoms from the supercell to form the defect, and that these errors are related to their atomic radius. Secondly, the charge models I constructed are found to be in good agreement with the predictions of classical electrostatics, corresponding to both bound and free charges. I derive a new analytic charge correction based on this model, but it produces smaller corrections than anticipated. Further calculations demonstrate that the remaining unexplained finite-size error is introduced by the finite-size dependence of the exchange-correlation energy of the screening bound charge, rather than on electrostatic grounds. Improved empirical corrections are constructed, which display differences between unrelaxed and relaxed defects. The developed arguments are extended to surface slab models, where the observed errors are found to be dependent on the shape of the supercell employed.
Supervisor: Shluger, A. Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available