  From first-principles calculations, we have studied the electronic and
magnetic structures of the ground state of LaOFeAs. The Fe spins are found to
be collinear antiferromagnetic ordered, resulting from the interplay between
the strong nearest and next-nearest neighbor superexchange antiferromagnetic
interactions. The structure transition observed by neutron scattering is shown
to be magnetically driven. Our study suggests that the antiferromagnetic
fluctuation plays an important role in the Fe-based superconductors. This sheds
light on the understanding of the pairing mechanism in these materials.
