  The first-principles electronic structure calculations made substantial
contribution to study of high $T_c$ iron-pnictide superconductors. By the
calculations, LaFeAsO was first predicted to be an antiferromagnetic semimetal,
and the novel bi-collinear antiferromagnetic order was predicted for
$\alpha$-FeTe. Moreover, based on the calculations the underlying mechanism was
proposed to be Arsenic-bridged antiferromagnetic superexchange interaction
between the next-nearest neighbor Fe moments. In this article, this physical
picture is further presented and discussed in association with the elaborate
first-principles calculations on LaFePO. The further discussion of origin of
the magnetism in iron-pnictides and in connection with superconductivity is
presented as well.
