  We theoretically study physical properties of the most promising color center
candidates for the recently observed single-photon emissions in hexagonal boron
nitride (h-BN) monolayers. Through our group theory analysis combined with
density functional theory (DFT) calculations we provide several pieces of
evidence that the electronic properties of the color centers match the
characters of the experimentally observed emitters. We calculate the
symmetry-adapted multi-electron wavefunctions of the defects using group theory
methods and analyze the spin-orbit and spin-spin interactions in detail. We
also identify the radiative and non-radiative transition channels for each
color center. An advanced ab-initio DFT method is then used to compute energy
levels of the color centers and their zero-phonon-line (ZPL) emissions. The
computed ZPLs, the profile of excitation and emission dipole polarizations, and
the competing relaxation processes are discussed and matched with the observed
emission lines. By providing evidence for the relation between single-photon
emitters and local defects in h-BN, this work provides the first steps towards
harnessing quantum dynamics of these color centers.
