  Defect states in 2D materials present many possible uses but both
experimental and computational characterization of their spectroscopic
properties is difficult. We provide and compare results from 13 DFT and ab
initio computational methods for up to 25 excited states of a paradigm system,
the VNCB defect in hexagonal boron nitride (h-BN). Studied include include: (i)
potentially catastrophic effects for computational methods arising from the
multi-reference nature of the closed-shell and open-shell states of the defect,
which intrinsically involves broken chemical bonds, (ii) differing results from
DFT and time-dependent DFT (TDDFT) calculations, (iii) comparison of cluster
models to periodic-slab models of the defect, (iv) the starkly differing
effects of nuclear relaxation on the various electronic states as broken bonds
try to heal that control the widths of photoabsorption and photoemission
spectra, (v) the effect of zero-point energy and entropy on free-energy
differences, (vi) defect-localized and conduction/valence band transition
natures, and (vii) strategies needed to ensure that the lowest-energy state of
a defect can be computationally identified.
