  A many-body theory of molecular junction transport based on nonequilibrium
Green's functions is developed, which treats coherent quantum effects and
Coulomb interactions on an equal footing. The central quantity of the many-body
theory is the Coulomb self-energy matrix $\Sigma_{\rm C}$ of the junction.
$\Sigma_{\rm C}$ is evaluated exactly in the sequential tunneling limit, and
the correction due to finite tunneling width is evaluated self-consistently
using a conserving approximation based on diagrammatic perturbation theory on
the Keldysh contour. Our approach reproduces the key features of both the
Coulomb blockade and coherent transport regimes simultaneously in a single
unified transport theory. As a first application of our theory, we have
calculated the thermoelectric power and differential conductance spectrum of a
benzenedithiol-gold junction using a semi-empirical $\pi$-electron Hamiltonian
that accurately describes the full spectrum of electronic excitations of the
molecule up to 8--10eV.
