  We determine and analyze the charge and spin density distributions of
nitrogen-vacancy (N-V) center in diamond for both the ground and excited states
by \emph{ab initio} supercell calculations. We show that the hyperfine tensor
of $^{15}$N nuclear spin is negative and strongly anisotropic in the excited
state, in contrast to previous models used extensively to explain electron spin
resonance measurements. In addition, we detect a significant redistribution of
the spin density due to excitation that has serious implications for the
quantum register applications of N-V center.
