  To examine the role of magnetism in superconductivity of iron-based
superconductors, we first present first-principles optical calculations on
three representative parent compounds: LaFeAsO, BaFe2As2 and LiFeAs. Both
nonmagnetic (NM) and spin-density wave (SDW) with striped antiferromagnetic
order are considered. Due to their layer structure, these compounds in NM
states show anomalously large optical anisotropy with a conductivity ratio
$\sigma_{zz}/\sigma{xx}$ as large as 313% in LaFeAsO and 530% in LiFeAs.
Interestingly the giant optical anisotropy is significantly suppressed by SDW.
On the other hand, the variable-cell-shape optimizations confirm that SDW does
drive an orthorhombic-distortion. The drive force is dependent on the local
magnetic moment on Fe sublattice, which is tuned by its bonding environment.
Based on these results, we discussed the delicate role of magnetism in
superconductivity.
