Convergence of defect energetics calculations

Determination of the chemical and spectroscopic natures of defects in materials such as hexagonal boron nitride (h-BN) remains a serious challenge for both experiment and theory. To establish basic needs for reliable calculations, we consider a model defect VNNB in h-BN in which a boron-for-nitrogen substitution is accompanied by a nitrogen vacancy, examining its lowest energy transition, (1)2A1 ← (1)2B1. This provides a relatively simple test system as open-shell and charge-transfer effects, which are difficult to model and can dominate defect spectroscopy, are believed to be small. We establish calculation convergence with respect to sample size using both cluster and 2D periodic models, convergence with respect to numerical issues such as the use of plane-wave or Gaussian basis set expansions, and convergence with respect to the treatment of electron correlation. The results strongly suggest that the poor performance of computational methods for defects of other natures arises through intrinsic methodological shortcomings.