  We analyze the effect of an external electric field on the electronic
structure of molecules which have been recently studied as molecular wires or
diodes. We use a self-consistent tight binding technique which provides results
in good agreement with ab initio calculations and which may be applied to a
large number of molecules. The voltage dependence of the molecular levels is
mainly linear with slopes intimately related to the electronic structure of the
molecules. We emphasize that the response to the applied voltage is an
important feature which governs the behavior of a molecular device.
