  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 basics needs for reliable
calculations, we consider a model defect $V_N N_B$ in h-BN in which a
boron-for-nitrogen substitution is accompanied by a nitrogen vacancy, examining
its lowest-energy transition, (1)2B1 to (1)2A1. This provides a relatively
simple test system as open-shell and charge-transfer effects, that 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 use of plane-wave or Gaussian-basis-set expansions, and
convergence with respect to the treatment of electron correlation. The results
strongly suggest that poor performance of computational methods for defects of
other natures arise through intrinsic methodological shortcomings.
