Towards a theory of electrical transport through atomic and molecular junctions

Present trends in the miniaturization of electronic devices suggest that ultimately single atoms and molecules may be used as electronically active elements in a variety of applications. In this context, there is an obvious request for a theory that can elucidate the transport mechanisms at the single-molecule scale, and in turn help in the future engineering of molecular devices. We present here a candidate to such a theory, which based on the combination of quantum chemistry methods and Green functions techniques. Our main goal in this work is to show how the electronic structure of single atoms and molecules controls the macroscopic electrical properties of the circuits in which they are used as building blocks. In particular, we review our work on three basic problems that have received a special experimental attention in the last years: (i) the conductance of a single-atom contact; (ii) the conductance of a hydrogen molecule; and (iii) the current through single organic molecules.