First principles simulation of neutral excitations in materials

The simulation of light activated processes in materials for energy sustainability and quantum information science requires a robust description of neutral excitations in complex heterogeneous systems. I will present a hierarchical modeling approach that enables us to simulate neutral excitations in materials with increasing complexity. First, I will discuss the simulation of excitons in large systems using density matrix perturbation theory, where the dielectric screening is evaluated from first principles with a finite field method or approximated by machine learning models. Second, I will discuss the simulation of neutral excitations in the presence of structural relaxations using the Huang-Rhys theory. Calculated photoluminescence spectra of point-defects are presented for diamond and silicon carbide, and carefully validated against experiment. Third, I will present the calculation of strongly-correlated neutral excitations of deep defects, e.g., color centers in diamond. A quantum embedding method that relies on input from density functional theory is used to generate an effective Hamiltonian that describes the low-lying excitations of the defect, and whose many-body eigenstates are obtained using configuration interaction. Finally, I will discuss opportunities for electronic structure calculations that are driven by emerging trends in the high-performance computing landscape, which include strategies to leverage exascale and quantum computing.