  Using linear-response density-functional theory, we obtain the magnetic
interactions in the several iron pnictides. The ground state has been found to
be non-collinear in FeSe, with a large continuum of nearly degenerate states
lying very close to the magnetic "striped" structure. The presence of
non-collinearity also seems to be a generic feature of iron pnictides when the
Fe moment is small. At small R_{Fe-Se} the system is itinerant: strong
frustration give rise to excess of spin entropy, long ranged interactions
create incommensurate orderings and strong biquadratic (or ring) couplings
violate the applicability of Heisenberg model. There is a smooth transition to
more localized behavior as R_{Fe-Se} increases: stable magnetic orbital order
develops which favor long range AFM stripe ordering with strongly anisotropic
in-plane exchange couplings. The stabilization of the stripe magnetic order is
accompanied by the inversion of the exchange coupling.
