  We present the first in a series of microscopic studies of electrical
transport through individual molecules with metallic contacts. We view the
molecules as ``heterostructures'' composed of chemically well-defined atomic
groups, and analyze the device characteristics in terms of the charge and
potential response of these atomic-groups to the perturbation induced by the
metal-molecule coupling and the applied electrical field, which are modeled
using a first-principles based self-consistent matrix Green's function (SCMGF)
method. As the first example, we examine the devices formed by attaching two
benzene-based molecular radicals--phenyl dithiol (PDT) and biphenyl dithiol
(BPD)--symmetrically onto two semi-infinite gold electrodes through the end
sulfur atoms.
