  Using density functional theory and many-body perturbation theory within a GW
approximation, we calculate the electronic structure of a metal-molecule
interface consisting of benzene diamine (BDA) adsorbed on Au(111). Through
direct comparison with photoemission data, we show that a conventional
G$_0$W$_0$ approach can underestimate the energy of the adsorbed molecular
resonance relative to the Au Fermi level by up to 0.8 eV. The source of this
discrepancy is twofold: a 0.7 eV underestimate of the gas phase ionization
energy (IE), and a 0.2 eV overestimate of the Au work function. Refinements to
self-energy calculations within the GW framework that account for deviations in
both the Au work function and BDA gas-phase IE can result in an interfacial
electronic level alignment in quantitative agreement with experiment.
