  The $V_N N_B$ defect in hexagonal boron nitride (h-BN), comprising a nitrogen
vacancy adjacent to a nitrogen-for-boron substitution, is modelled in regard to
its possible usefulness in a nanophotonics device. The modelling is done on
both a simple model compound and on a 2D periodic representation of the defect,
considering its magnetic and spectroscopic properties. The electronic
distribution in $V_N N_B$ excited states is very open-shell in nature, and to
deal with this two new computational methods are developed: one allows standard
density-functional theory (DFT) calculations to be employed to evaluate state
energies, the other introduces techniques needed to apply the VASP
computational package to these and many other problems involving excited
states. Also of general use, results from DFT calculations are then calibrated
against those from ab initio methods, seeking robust computational schemes.
These innovations allow 45 electronic states of the defect in its neutral, +1
and -1 charged forms to be considered. The charged forms of the defect are
predicted to display properties of potential interest to nanophotonics.
