  We analyze the effect of a gate on the conductance of molecules by separately
evaluating the gate-induced polarization and the potential shift of the
molecule relative to the leads. The calculations use ab initio density
functional theory combined with a Green function method for electron transport.
For a general view, we study several systems: (1) atomic chains of C or Al
sandwiched between Al electrodes, (2) a benzene molecule between Au leads, and
(3) (9,0) and (5,5) carbon nanotubes. We find that the polarization effect is
small because of screening, while the effect of the potential shift is
significant, providing a mechanism for single-molecule transistors.
