  The current-voltage characteristics in Langmuir-Blodgett monolayers of
\gamma-hexadecylquinolinium tricyanoquinodimethanide (C16H33Q-3CNQ) sandwiched
between Al or Au electrodes is calculated, combining ab initio and
self-consistent tight binding techniques. The rectification current depends on
the position of the LUMO and HOMO relative to the Fermi levels of the
electrodes as in the Aviram-Ratner mechanism, but also on the profile of the
electrostatic potential which is extremely sensitive to where the electroactive
part of the molecule lies in the monolayer. This second effect can produce
rectification in the direction opposite to the Aviram-Ratner prediction.
