Analyzing Electronic Correlation Effects in Molecules and Semiconductor Point Defects from First Principles Beyond Density Functional Theory

Abstract

Transition-metal based molecules and point defects in wide-bandgap semiconductors have been of particular interest lately due to their potential quantum information science applications. To accurately calculate the electronic properties of these systems from first principles, it is important to appropriately account for electronic correlation effects. Density functional theory (DFT) has been one of the most popular methods to perform calculations on correlated systems, due to the combination of numerical efficiency and precision in many applications. However, traditional DFT methods fail to accurately calculate the important electronic and magnetic properties, such as bandgaps in semiconductors or magnetic ordering in defects, to name a few. This dissertation focuses on two areas in which traditional DFT methods are likely to produce inaccurate predictions. The first area is connected to an error that is intrinsic to most DFT formulations due to the approximate nature of the exchange-correlation functional, known as the self-interaction error (SIE). It is known to cause the underestimation of bandgaps in solids, underestimated reaction barriers in molecules, and incorrect dissociation curves. The second area is the case where the ground and/or excited states are described by multiconfigurational wavefunctions rather than a single Slater determinant. Chapter 1 of the thesis provides a brief overview of various electronic-structure methods, as well as the Fermi-Löwdin Orbital Self-Interaction Correction method (FLOSIC) which is used to remedy the SIE in DFT. Chapter 2 reports on the application of the FLOSIC-DFT on a Cu-based molecule, and its effects on the predicted electronic properties. Chapter 3 describes the application of the FLOSIC-DFT to the computation of the hyperfine coupling terms, which are crucial for the realization of spin qubits and for interpreting electron paramagnetic resonance experiments. Chapter 4 turns to the application of a multiconfigurational method to describe the electronic properties of a silicon vacancy in silicon carbide, a potential point-defect qubit.