First principles studies of quantum point defects

A robust description of excited states for complex heterogeneous systems is needed to model optically activated processes in materials. We focus on simulations of point defects in diamond and silicon carbide, which are of interest for the realization of quantum technologies. We use a hierarchical modeling approach that relies on the combination of time-dependent density functional theory (TDDFT), many-body perturbation theory, and multi-reference methods. We present a formulation of spin-conserving and spin-flip TDDFT, including the calculation of analytical forces, which allows for efficient calculations of excited state properties of solid-state systems with hundreds to thousands of atoms. To describe strongly correlated neutral excitations in point-defects embedded in periodic crystals we use full configuration interaction (FCI) embedded in DFT+GW, i.e., we derive an effective Hamiltonian that describes the low-lying excitations of the defect using the quantum defect embedding theory (QDET). These examples benefit from the use of the latest developments in high-performance computing architectures, which include pre-exascale capable machines and quantum processors.