  We develop a \pi-electron effective field theory (\pi-EFT) wherein the
two-body Hamiltonian for a \pi-electron system is expressed in terms of three
effective parameters: the \pi-orbital quadrupole moment, the on-site repulsion,
and a dielectric constant. As a first application of this \pi-EFT, we develop a
model of screening in molecular junctions based on image multipole moments, and
use this to investigate the reduction of the HOMO-LUMO gap of benzene. Beyond
this, we also use \pi-EFT to calculate the differential conductance spectrum of
the prototypical benzenedithiol-Au single-molecule junction and the
\pi-electron contribution to the van der Waals interaction between benzene and
a metallic electrode.
