The energy level alignment of the two organic materials forming the TTF-TCNQ
interface is analyzed by means of a local orbital DFT calculation, including an
appropriate correction for the transport energy gaps associated with both
materials. These energy gaps are determined by a combination of some
experimental data and the results of our calculations for the difference
between the TTF_{HOMO} and the TCNQ_{LUMO} levels. We find that the interface
is metallic, as predicted by recent experiments, due to the overlap (and charge
transfer) between the Density of States corresponding to these two levels,
indicating that the main mechanism controlling the TTF-TCNQ energy level
alignment is the charge transfer between the two materials. We find an induced
interface dipole of 0.7 eV in good agreement with the experimental evidence. We
have also analyzed the electronic properties of the TTF-TCNQ interface as a
function of an external bias voltage \Delta, between the TCNQ and TTF crystals,
finding a transition between metallic and insulator behavior for \Delta~0.5 eV
