I will discuss two topics:

(1) In future "molecular electronics", single molecules contacted by metal leads would serve as building blocks in nanoscale devices. Only recently have accurate experimental and theoretical techniques for studying such systems become available. A particularly interesting molecule is azobenzene, a light-driven molecular switch which has been shown to function even when adsorbed on metal surfaces [1,2]. Using a DFT-based method with model self-energy corrections, we have calculated the zero-bias conductance of Au-molecule-Au junctions of trans- and cis-azobenzene, finding a reasonable ON/OFF ratio. Calculations on a set of various organic molecules with a generalized version of this methodology has shown quantitative agreement with experimental measurements by the STM break-junction technique [3].

(2) Non-linear optics is an important characterization tool for surface science, chemistry, and biology, as well as technologically useful in manipulating light for optics and communications. Current non-linear materials are expensive, hard-to-grow inorganic crystals, so there has been a long-running search for good non-linear organic molecules. Moreover, in testing such molecules in solution, as well as in the characterization applications, there are fundamental questions unanswered about the effect of the condensed-phase environment on non-linear optical properties. To address this question, we study liquid chloroform, a solvent commonly used in experiments, using time-dependent density-functional theory in the Octopus code. We assess the basis sets required for convergence and the effect of structural perturbations in the gas phase [4], and then have developed methods to allow a full liquid calculation and extraction of "local field factors" which related the field felt by a molecule to the applied field.
[1] MJ Comstock, N Levy, A Kirakosian, J Cho, F Lauterwasser, JH Harvey, DA Strubbe, JMJ Fréchet, D Trauner, SG Louie, and MF Crommie, Phys. Rev. Lett. 99, 038301 (2007).
[2] MJ Comstock, DA Strubbe, L Berbil-Bautista, N Levy, J Cho, D Poulsen, JMJ Fréchet, SG Louie, and MF Crommie, Phys. Rev. Lett. 104, 178301 (2010).
[3] SY Quek, DA Strubbe, HJ Choi, SG Louie, and JB Neaton, in preparation (2010).
[4] F Vila, D Strubbe, Y Takimoto, X Andrade, A Rubio, SG Louie, JJ Rehr, accepted to J. Chem. Phys. (2010)\\