Insights and challenges of applying the GW method to transition metal oxides

Koopmans Spectral Functionals in Periodic Boundary Conditions

Koopmans spectral functionals aim to describe simultaneously ground-state properties and charged excitations of atoms, molecules, nanostructures, and periodic crystals. This is achieved by augmenting standard density functionals with simple but physically motivated orbital-density-dependent corrections. These corrections act on a set of localized orbitals that, in periodic systems, resemble maximally localized Wannier functions. At variance with the original, direct supercell implementation (Phys. Rev. X 2018, 8, 021051), we discuss here (i) the complex but efficient formalism required for a periodic boundary code using explicit Brillouin zone sampling and (ii) the calculation of the screened Koopmans corrections with density functional perturbation theory. In addition to delivering improved scaling with system size, the present development makes the calculation of band structures with Koopmans functionals straightforward. The implementation in the open-source Quantum ESPRESSO distribution and the application to prototypical insulating and semiconducting systems are presented and discussed.

The Operando Optical Spectrum of Hematite during Water Splitting through a GW-BSE Calculation

Hematite is a possible photoanode for photoelectrochemical cells (PECs) that is widely studied using density functional theory (DFT). In this paper we perform more accurate calculations of the absorption spectrum of hematite using the one-shot Green's function (G₀W₀) and Bethe-Salpeter equation (BSE) methods, which take excited states into account and compare the spectrum to experimental data. We found a match between our calculations and the observed absorption spectra in peak locations. Furthermore, there is anisotropy of the absorption spectra that is concurrent with the crystal structure. We also calculated the absorption spectrum of hematite intermediates during catalysis of the oxygen evolution reaction to better understand which intermediate is dominant during the reaction and the contribution of excited states to catalysis. The *O intermediate was found to be the most optically and chemically dominant species during catalysis.

Accuracy of dielectric-dependent hybrid functionals in the prediction of optoelectronic properties of metal oxide semiconductors: a comprehensive comparison with many-body GW and experiments

Understanding the electronic structure of metal oxide semiconductors is crucial to their numerous technological applications, such as photoelectrochemical water splitting and solar cells. The needed experimental and theoretical knowledge goes beyond that of pristine bulk crystals, and must include the effects of surfaces and interfaces, as well as those due to the presence of intrinsic defects (e.g. oxygen vacancies), or dopants for band engineering. In this review, we present an account of the recent efforts in predicting and understanding the optoelectronic properties of oxides using ab initio theoretical methods. In particular, we discuss the performance of recently developed dielectric-dependent hybrid functionals, providing a comparison against the results of many-body GW calculations, including G ₀ W ₀ as well as more refined approaches, such as quasiparticle self-consistent GW. We summarize results in the recent literature for the band gap, the band level alignment at surfaces, and optical transition energies in defective oxides, including wide gap oxide semiconductors and transition metal oxides. Correlated transition metal oxides are also discussed. For each method, we describe successes and drawbacks, emphasizing the challenges faced by the development of improved theoretical approaches. The theoretical section is preceded by a critical overview of the main experimental techniques needed to characterize the optoelectronic properties of semiconductors, including absorption and reflection spectroscopy, photoemission, and scanning tunneling spectroscopy (STS).