For molecular electronics, Boltzmann's equation is no longer valid for simulating device characteristics. We present the first fully ab initio simulation of a moelcular device that has already been studied experimentally, namely a benze4ne-1,4-dithiolate molecule between gold electrodes. The theretical I-V curve has the same overall shape as the experimental curve-reflecting the electronic structure of the molecule in the presence of the electric field - but the absolute vlaue of the current is very sensitive to contact chemistry ad geoemtry. In particular the presence of a single gold atom between the molecule and the electrode surface reduces the conductance by more than an order of magnitude. Replacement of the single gold awtom by an aluminum atom, whose p orbitals couple more effectively to the molecule's pi orbitals, increases the conductance by about an order of magnitude. We have also studied the polarization effects induced by a third therminal (gate) on the I-V characteristics of the avove deivce. In particular, we have found that current gain due to the gate bias can be achieved at reasonable gate fields. Finally the effect of current-induced forces on the device will be discussed.
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