Electron transport through molecules: Gate-induced polarization and potential shift

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: s1d atomic chains of C or Al sandwiched between Al electrodes, s2d a benzene molecule between Au leads, and s3d s9,0d and s5,5d 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 singlemolecule transistors.