  The split silicon-vacancy defect (SiV) in diamond is an electrically and
optically active color center. Recently, it has been shown that this color
center is bright and can be detected at the single defect level. In addition,
the SiV defect shows a non-zero electronic spin ground state that potentially
makes this defect an alternative candidate for quantum optics and metrology
applications beside the well-known nitrogen-vacancy color center in diamond.
However, the electronic structure of the defect, the nature of optical
excitations and other related properties are not well-understood. Here we
present advanced \emph{ab initio} study on SiV defect in diamond. We determine
the formation energies, charge transition levels and the nature of excitations
of the defect. Our study unravel the origin of the dark or shelving state for
the negatively charged SiV defect associated with the 1.68-eV photoluminescence
center.
