  We study the role of non-locality of exchange in a neutral zigzag graphene
nanoribbon within the $\pi$-orbital unrestricted Hartree-Fock approximation.
Within this theory we find that the magnetic features are further stabilized
for both the intra-edge and inter-edge exchange compared to mean field theories
of zigzag ribbon based on local exchange (like the Hubbard model or the
ab-initio Local Density Approximation). The inter-edge exchange produces an
enhancement of the band-gap of the magnetic ground-state solutions. The effect
of this enhanced exchange on the edge states can not be satisfactorily achieved
by a local interaction with renormalized parameters.
