  Based on spin-density functional theory we calculate the electronic structure
of a tunnel junction consisting of two magnetic Fe layers separated by an
insulating vacuum barrier selfconsistently. For the conductance the Landauer
formula is evaluated in the ballistic limit as function of the magnetic
configuration. Based on these conductances the tunnel magnetoresistance (TMR)
ratio is obtained. We investigate the relation between TMR ratio and spin
polarization of the electronic structure at the metal/insulator interface.
