  Hexagonal boron nitride (hBN) has recently emerged as a fascinating platform
for room-temperature quantum photonics due to the discovery of robust visible
light single-photon emitters. In order to utilize these emitters, it is
necessary to have a clear understanding of their atomic structure and the
associated excitation processes that give rise to this single photon emission.
Here we perform density-functional theory (DFT) and constrained DFT
calculations for a range of hBN point defects in order to identify potential
emission candidates. By applying a number of criteria on the electronic
structure of the ground state and the atomic structure of the excited states of
the considered defects, and then calculating the Huang-Rhys (HR) factor, we
find that the CBVN defect, in which a carbon atom substitutes a boron atom and
the opposite nitrogen atom is removed, is a potential emission source with a HR
factor of 1.66, in good agreement with the experimental HR factor. We calculate
the photoluminescence (PL) line shape for this defect and find that it
reproduces a number of key features in the the experimental PL lineshape.
