Conformation dependence of molecular conductance:: chemistry versus geometry

Recent experiments by Venkataraman et al ( 2006 Nature 442 904) on a series of molecular wires with varying chemical compositions revealed a linear dependence of the conductance on cos2 theta, where theta is the angle of twist between neighbouring aromatic rings. To investigate whether or not this dependence has a more general applicability, we present a first-principles theoretical study of the transport properties of this family of molecules as a function of the chemical composition, conformation and the contact atom and geometry. If the Fermi energy E-F lies within the HOMO-LUMO ( highest occupied molecular orbital-lowest unoccupied molecular orbital) gap, then we reproduce the above experimental results. More generally, however, if EF is located within either the LUMO or the HOMO states, the presence of resonances destroys the linear dependence of the conductance on cos(2) theta and gives rise to non-monotonic behaviour associated with the level structure of the different molecules. Our results suggest that the above experiments provide a novel method for extracting spectroscopic information about molecules contacted to electrodes.