  The negatively charged nitrogen-vacancy defect ($N-V^-$ center) in diamond is
an important atomic-scale structure that can be used as a qubit in quantum
computing and as a marker in biomedical applications. Its usefulness relies on
the ability to optically excite electrons between well-defined gap states,
which requires clear and detailed understanding of the relevant states and
excitation processes. Here we show that by using hybrid
density-functional-theory calculations in a large supercell we can reproduce
the zero-phonon line and the Stokes and anti-Stokes shifts, yielding a complete
picture of the spin-conserving excitation of this defect.
