  The accurate determination of the lowest electron attachment ($EA$) and
ionization ($IP$) energies for molecules embedded in molecular junctions is
important for correctly estimating, \emph{e.g.}, the magnitude of the currents
($I$) or the biases ($V$) where an $I-V$-curve exhibits a significant non-Ohmic
behavior. Benchmark calculations for the lowest electron attachment and
ionization energies of several typical molecules utilized to fabricate
single-molecule junctions characterized by n-type conduction (4,4'-bipyridine,
1,4-dicyanobenzene, and 4,4'-dicyano-1,1'-biphenyl) and p-type conduction
(benzenedithiol, biphenyldithiol, hexanemonothiol, and hexanedithiol] based on
the EOM-CCSD (equation-of-motion coupled-cluster singles and doubles)
state-of-the-art method of quantum chemistry are presented. They indicate
significant differences from the results obtained within current approaches to
molecular transport. The present study emphasizes that, in addition to a
reliable quantum chemical method, basis sets much better than the ubiquitous
double-zeta set employed for transport calculations are needed. The latter is a
particularly critical issue for correctly determining $EA$'s, which is
impossible without including sufficient diffuse basis functions. The spatial
distribution of the dominant molecular orbitals (MO's) is another important
issue, on which the present study draws attention, because it sensitively
affects the MO-energy shifts $\Phi$ due to image charges formed in electrodes.
The present results cannot substantiate the common assumption of a point-like
MO midway between electrodes, which substantially affects the actual
$\Phi$-values.
