### Browsing by Author "Shang, Honghui"

Now showing 1 - 3 of 3

###### Results Per Page

###### Sort Options

Item All-electron, real-space perturbation theory for homogeneous electric fields: Theory, implementation, and application within DFT(2018-07-01) Shang, Honghui; Raimbault, Nathaniel; Rinke, Patrick; Scheffler, Matthias; Rossi, Mariana; Carbogno, Christian; Department of Applied Physics; Computational Electronic Structure Theory; Fritz-Haber-Institut der Max-Planck-GesellschaftWithin density-functional theory, perturbation theory (PT) is the state-of-the-art formalism for assessing the response to homogeneous electric fields and the associated material properties, e.g., polarizabilities, dielectric constants, and Raman intensities. Here, we derive a real-space formulation of PT and present an implementation within the all-electron, numeric atom-centered orbitals electronic structure code FHI-aims that allows for massively parallel calculations. As demonstrated by extensive validation, we achieve a rapid computation of accurate response properties of molecules and solids. As an application showcase, we present harmonic and anharmonic Raman spectra, the latter obtained by combining hundreds of thousands of PT calculations with ab initio molecular dynamics. By using the PBE exchange-correlation functional with many-body van der Waals corrections, we obtain spectra in good agreement with experiment especially with respect to lineshapes for the isolated paracetamol molecule and two polymorphs of the paracetamol crystal.Item Electron-phonon coupling in d-electron solids: A temperature-dependent study of rutile TiO2 by first-principles theory and two-photon photoemission(American Physical Society, 2019-12-05) Shang, Honghui; Argondizzo, Adam; Tan, Shijing; Zhao, Jin; Rinke, Patrick; Carbogno, Christian; Scheffler, Matthias; Petek, Hrvoje; Department of Applied Physics; Computational Electronic Structure Theory; Fritz-Haber-Institut der Max-Planck-Gesellschaft; University of Pittsburgh; University of Science and Technology of ChinaRutile TiO2 is a paradigmatic transition-metal oxide with applications in optics, electronics, photocatalysis, etc., that are subject to pervasive electron-phonon interaction. To understand how energies of its electronic bands, and in general semiconductors or metals where the frontier orbitals have a strong d-band character, depend on temperature, we perform a comprehensive theoretical and experimental study of the effects of electron-phonon (e-p) interactions. In a two-photon photoemission (2PP) spectroscopy study we observe an unusual temperature dependence of electronic band energies within the conduction band of reduced rutile TiO2, which is contrary to the well-understood sp-band semiconductors and points to a so far unexplained dichotomy in how the e-p interactions affect differently the materials where the frontier orbitals are derived from the sp- and d orbitals. To develop a broadly applicable model, we employ state-of-the-art first-principles calculations that explain how phonons promote interactions between the Ti-3d orbitals of the conduction band within the octahedral crystal field. The characteristic difference in e-p interactions experienced by the Ti-3d orbitals of rutile TiO2 crystal lattice are contrasted with the more familiar behavior of the Si-2s orbitals of stishovite SiO2 polymorph, in which the frontier 2s orbital experiences a similar crystal field with the opposite effect. The findings of this analysis of how e-p interactions affect the d- and sp-orbital derived bands can be generally applied to related materials in a crystal field. The calculated temperature dependence of d-orbital derived band energies agrees well with and explains the temperature-dependent inter-d-band transitions recorded in 2PP spectroscopy of TiO2. The general understanding of how e-p interactions affect d-orbital derived bands is likely to impact the understanding of temperature-dependent properties of highly correlated materials.Item Lattice dynamics calculations based on density-functional perturbation theory in real space(2017) Shang, Honghui; Carbogno, Christian; Rinke, Patrick; Scheffler, Matthias; Department of Applied Physics; Computational Electronic Structure Theory; Fritz-Haber-Institut der Max-Planck-GesellschaftA real-space formalism for density-functional perturbation theory (DFPT) is derived and applied for the computation of harmonic vibrational properties in molecules and solids. The practical implementation using numeric atom-centered orbitals as basis functions is demonstrated exemplarily for the all-electron Fritz Haber Institute ab initio molecular simulations (FHI-aims) package. The convergence of the calculations with respect to numerical parameters is carefully investigated and a systematic comparison with finite-difference approaches is performed both for finite (molecules) and extended (periodic) systems. Finally, the scaling tests and scalability tests on massively parallel computer systems demonstrate the computational efficiency.