  Currents across thin insulators are commonly taken as single electrons moving
across classically forbidden regions; this independent particle picture is
well-known to describe most tunneling phenomena. Examining quantum transport
from a different perspective, i.e., by explicit treatment of electron-electron
interactions, we evaluate different single particle approximations with
specific application to tunneling in metal-molecule-metal junctions. We find
maximizing the overlap of a Slater determinant composed of single particle
states to the many-body current-carrying state is more important than energy
minimization for defining single particle approximations in a system with open
boundary conditions. Thus the most suitable single particle effective potential
is not one commonly in use by electronic structure methods, such as the
Hartree-Fock or Kohn-Sham approximations.
