This dissertation belongs to the domain of computational material physics with a focus on theory and application of density functional theory (DFT). The main focus is on the semiclassical description of atoms, and its use in improving kinetic energy functionals for orbital-free density functional theory (OFDFT). The dissertation comprises four publications, three of which are targeting the study and improvement of OFDFT. The fourth publication is a density functional theory study of wide band-gap semiconductor Ga2O3. OFDFT is an approach in which the physical properties of materials are direct functionals of electronic density in the spirit of Hohenberg-Kohn theorems. Relying only on the electronic density makes even quite large systems computationally tractable. Naturally, there is a trade-off between accuracy and speed for realistic orbital-free approximations when compared to the Kohn-Sham scheme, which is more accurate but uses computationally more expensive non-interacting orbitals to predict the properties of a material. The nature of the orbital-free approximation is semiclassical, where the Pauli principle is only roughly taken into account. We use the generalized gradient approximation (GGA) to develop a new kinetic energy functional form RATIONAL for OFDFT, which unifies previous results on GGA kinetic energy functionals. The parameters of the RATIONAL functional are obtained from the semiclassical description of atomic systems, resulting in a well-performing functional for simple solids. We study the limit of large atomic numbers, large-Z limit, for the ionization potential of atoms and lattice constants of simple solids. Remarkably it is found that both Kohn-Sham and orbital-free models have finite large-Z limit for the lattice constants. The Englert-Schwinger (ES) model is an orbital-free model of the atom, which uses potential functionals for essential approximations. The model sidesteps a theoretical flaw, which is the inadequate description of electrons near the nucleus, found in other orbital-free models. We solve the ES model self-consistently and analyze it by comparing it to Kohn-Sham results and other orbital-free models. We find that ES model provides a better description of ionization potential in the large-Z limit than other orbital-free models. As an application of DFT, we perform a defect study of wide band-gap semiconductor Ga2O3. There is a growing interest in solid-state neutron detection, e.g. for nuclear safety, to replace the current 3He detectors, which are expensive and bulky. We study the defect structure of Ga2O3 to assess if it is possible to dope it with neutron active element 10B . We find that for a broad range of chemical environments, it is feasible to introduce electronically inactive 10B into Ga2O3.
|Translated title of the contribution||Tiheysfunktionaaliteorian approksimaatiota semiklassisesta lähestymistavasta|
|Publication status||Published - 2020|
|MoE publication type||G5 Doctoral dissertation (article)|
- density functional theory
- gallium oxide