Theory of the ground state spin of the NV- center in diamond: I. Fine   structure, hyperfine structure, and interactions with electric, magnetic and   strain fields
  The ground state spin of the negatively charged nitrogen-vacancy center in diamond has been the platform for the recent rapid expansion of new frontiers in quantum metrology and solid state quantum information processing. In ambient conditions, the spin has been demonstrated to be a high precision magnetic and electric field sensor as well as a solid state qubit capable of coupling with nearby nuclear and electronic spins. However, in spite of its many outstanding demonstrations, the theory of the spin has not yet been fully developed and there does not currently exist thorough explanations for many of its properties, such as the anisotropy of the electron g-factor and the existence of Stark effects and strain splittings. In this work, the theory of the ground state spin is fully developed for the first time using the molecular orbital theory of the center in order to provide detailed explanations for the spin's fine and hyperfine structures and its interactions with electric, magnetic and strain fields. 
M.W. Doherty, F. Dolde, H. Fedder, F. Jelezko, J. Wrachtrup, N.B.   Manson and L.C.L. Hollenberg
cond-mat.mtrl-sci