  Extensive photochemical and spectroscopic properties of the $V_B^-$ defect in
hexagonal boron nitride are calculated, concluding that the observed
photoemission associated with recently observed optically-detected magnetic
resonance is most likely of (1)3E" to (1)3A2' origin. Rapid intersystem
crossing from the defect's triplet to singlet manifolds explains the observed
short excited-state lifetime and very low quantum yield. New experimental
results reveal smaller intrinsic spectral bandwidths than previously
recognized, interpreted in terms spectral narrowing and zero-phonon-line
shifting induced by the Jahn-Teller effect. Different types of computational
methods are applied to map out the complex triplet and singlet defect
manifolds, including the doubly ionised formulation of the equation-of-motion
coupled-cluster theory that is designed to deal with the open-shell nature of
defect states, and mixed quantum-mechanics/molecular-mechanics schemes enabling
5763-atom simulations. Two other energetically feasible spectral assignments
from amongst the singlet and triplet manifolds are considered, but ruled out
based on inappropriate photochemical properties.
