Excited-state geometry relaxation of point defects in monolayer hexagonal boron nitride

Optical excitation and deexcitation processes are governed by the dependence of the involved energy levels on the atom positions. This geometry dependence is calculated here for several defects in h-BN, e.g., CBVN, within constrained density functional theory and many-body perturbation theory. The investigated Stokes shifts show distinct differences compared to experiments. For the Tamm-Dancoff approximation, an approximation frequently employed in the calculation of exciton energies in crystalline solids, the authors find small but significant influence for defect systems.