Excitation Energies of Defects in Hexagonal Boron Nitride via an Embedding Method using Auxilliary-Field Quantum Monte Carlo

Defects in 2-d hexagonal boron nitride (h-BN) have shown promise for applications as color centers as well as possible qubit realizations. Producing these applications in a 2-d system is especially tantalizing because of the additional control available in manufacturing 2-d systems. However, the types and properties of defects in BN are varied, difficult to characterize experimentally, and different approximate first-principles treatments yield qualitatively different predictions for a given defect. We applied high-accuracy first-principles auxiliary-field quantum Monte Carlo (AFQMC) calculations to predict vertical excitations and intersystem crossings for the C_BV_N defect in h-BN. We were able to reach sufficiently large supercell sizes by applying an embedding method. Correlations within a given radius around the defect are treated with AFQMC, and this calculation is embedded in a bulk treated with independent-electron theory. The favorable scaling of AFQMC allowed us to expand the radius defining the correlated orbitals until all quantities were well-converged. This approach opens new possibilities for accurate many-body treatment of defect systems using embedding in combination with AFQMC.