  Estimating the relative alignment between the frontier molecular orbitals
that dominates the charge transport through single-molecule junctions
represents a challenge for theory. This requires approaches beyond the widely
employed framework provided by the density functional theory, wherein the
Kohn-Sham "orbitals" are treated as if they were real molecular orbitals, which
is not the case. In this paper, we report results obtained by means of quantum
chemical calculations, including the EOM-CCSD (equation-of-motion
coupled-cluster singles and doubles), which is the state-of-the-art of quantum
chemistry for medium-size molecules like those considered here. These
theoretical results are validated against data on the molecular orbital energy
offset relative to the electrodes' Fermi energy extracted from experiments for
junctions based on 4,4'-bipyridine and 1,4-dicyanobenzene.
