  To study the microscopic electronic and magnetic interactions in the
substoichiometric iron chalcogenide FeSe$_{1-x}$ which is observed to
superconduct at x~1/8 up to $T_c$=27 K, we use first principles methods to
study the Se vacancy in this nearly magnetic FeSe system. The vacancy forms a
ferrimagnetic cluster of eight Fe atoms, which for the ordered x=1/8 alloy
leads to half metallic conduction. Similar magnetic clusters are obtained for
FeTe$_{1-x}$ and for BaFe$_2$As$_2$ with an As vacancy, although neither of
these are half metallic. Based on fixed spin density results, we suggest the
low energy excitations in FeSe$_{1-x}$ are antiparamagnon-like with short
correlation length.
