  In this contribution, we calculate in a self-consistent way the ballistic
transmission as a function of energy of one Fe/MgO (001) single-barrier and one
double-barrier tunnel junction, relating them to their electronic structure.
The transmission spectra of each kind of junction is calculated at different
applied bias voltages. We focus on the impact that bias has on the resonant
tunneling mediated by surface and quantum well states. The calculations are
done in the coherent regime, using a combination of density functional theory
and non-equilibrium Green's functions, as implemented in the {\it ab initio}
code {\it SMEAGOL}. We conclude that, for both kinds of junction, the
transmission functions depend on the applied bias voltage. In the
single-barrier junction, transport mediated by resonant Fe minority surface
states is rapidly destroyed by bias. In the double-barrier junction, the
appearance of resonant tunneling through majority quantum well states is
strongly affected by bias.
