First-principles studies of electrical transport in nanoscale molecular junctions

Understanding the conductance of individual molecular junctions is a forefront topic in theoretical nanoscience. The development of a general, efficient atomistic approach for treating an open system out of equilibrium with good accuracy, and then using it to inform experiment, is a significant open challenge in the field. Here I will describe studies where first-principles techniques, based on density functional theory (DFT) and beyond, are used to investigate some of the fundamental issues associated with single-molecule transport measurements. After a brief summary of previous work, a DFT-based scattering-state approach is presented and applied to H$_2$ and amine-Au linked molecular junctions [1], two systems for which there exist reliable data [2]. Similar to most ab initio studies, we rely on a Landauer approach within DFT for junction conductance. Using this framework, which has proven relatively accurate for metallic point contacts, good agreement with experiment is obtained for the H$_2$ conductance. For amine-Au linked junctions, however, the computed conductance is significantly larger than that measured,although structural trends are reproduced by the calculations. To explore this further, we draw on GW calculations of a prototypical metal-molecule contact, benzene on graphite, where interfacial polarization effects are found to drastically modify frontier orbital energies [3]. A physically motivated model self-energy correction is developed from our GW calculations,applied to the amine case, and shown to quantitatively explain the discrepancy with experiment. The importance of many-electron corrections beyond DFT for accurately computing molecular conductance and understanding experiments is thoroughly discussed. [1] S. Y. Quek {\it et al.}, Nano Lett {\bf 7}, 3482 (2007); K. H. Khoo {\it et al.}, submitted (2007). [2] R. Smit {\it et al.}, Nature {\bf 419}, 906 (2002); L. Venkataraman {\it et al.}, Nature {\bf 442} ,904 (2006). [3] J. B. Neaton {\it et al.}, Phys. Rev. Lett. {\bf 97}, 216405 (2006).