  We report evidence that the experimentally observed small deformation of
antiferromagnetic NiO modifies the symmetry of the crystal in such a way that
the antiferromagnetic state becomes an eigenstate of the electronic
Hamiltonian. This deformation closely resembles a rhombohedral contraction, but
does not possess the perfect symmetry of a trigonal (rhombohedral) space group.
We determine the monoclinic base-centered magnetic space group of the
antiferromagnetic structure within the deformed crystal which is strongly
influenced by the time-inversion symmetry of the Hamiltonian. The
antiferromagnetic state is evidently stabilized by a nonadiabatic atomic-like
motion of the electrons near the Fermi level. This atomic-like motion is
characterized by the symmetry of the Bloch functions near the Fermi level and
provides in NiO a perfect basis for a Mott insulator.
