Effective field theory of interacting pi electrons in molecular junctions

We present an effective field theory that allows the two-body Hamiltonian for a $\pi$ electron system to be expressed in terms of three effective parameters: the $\pi$ orbital quadrupole moment, the on-site repulsion, and a dielectric constant. As an application of this theory, we present a model of screening in single-molecule junctions based on the image charge method, and use this technique to calculate the van der Waals interaction between a neutral molecule and a metallic conductor.