  We study whether Fermi-surface (FS) nesting can give rise to high-temperature
superconductivity in iron pnictides. Starting with ab initio construction of an
effective four-orbital model, we employ the fluctuation-exchange approximation
to show that FS does not necessarily favor the stripe antiferromagnetic order
observed in experiments, especially for realistic electronic correlations. If
superconductivity in iron pnictides is magnetically mediated and has
fully-gapped sign-reversing s-wave symmetry, our results suggest that the
pairing interaction does not arise only from FS nesting and exchange
interactions between local moments in the Fe 3d orbitals may play a crucial
role.
