  We discuss the general form of the transmission spectrum through a molec-
ular junction in terms of the Green function of the isolated molecule. By
introducing a tight binding method, we are able to translate the Green func-
tion properties into practical graphical rules for assessing beforehand the
possible existence of antiresonances in an energy range for a given choice of
connecting sites. The analysis is exemplified with a benzene molecule under a
hypothetical local gate, which allows one to continuously tune the on-site
energy of single atoms, for various connection topologies and gate positions.
