Exploring the lead dependence of single-molecule conductance from first principles: The case of H$_{2}$ molecular junctions

Although the transport properties of several single-molecule junctions have now been reported, only a few studies have systematically examined the sensitivity of the junction conductance to the choice of metallic contacts. Recent break-junction experiments have revealed significantly lower conductance for H$_{2}$ molecular junctions when Pt leads were replaced with Pd,$^{1,2}$ suggesting a dramatic difference in electronic coupling between the molecule and lead. In this work, we examine this coupling directly by computing the conductance of H$_{2}$ with several different metallic contacts using an \textit{ab-initio} scattering state approach$^{3 }$ based on density functional theory.$^{ }$We find that by substituting Pt with Pd leads, the low-bias electron transport crosses over from a ballistic to an off-resonance tunneling regime, leading to a conductance smaller than unity in agreement with experiments. The extent to which substituting different leads may be used to tune the transport properties of this and other simple single-molecule junctions will be discussed. This work was supported by the NSF Grant No. DMR04-39768 and U.S. DOE Contract No. DE-AC03-76SF00098. [1] R. H. M. Smit \textit{et al.}, Nature (London) \textbf{419}, 906 (2002). [2] Sz. Csonka \textit{et al.}, Phys. Rev. Lett. \textbf{93}, 016802 (2004). [3] H.J. Choi, M.L. Cohen and Steven G. Louie, to be published.