  Noble metals break preferably exposing (111)-oriented surfaces, that host
Shockley type surface states (SSs). Nevertheless, the relevance of SSs on the
electrical properties of gold-based molecular junctions has not been explored
in detail yet. Here, we present ab initio simulations that show how the gold
(111) SS, that lies approximately 0.5 eV below the Fermi energy, is key to
determining correctly the electrical and thermoelectric response of the above
junctions. We show how the ability to shift in a controlled way the energy
position of gold SS enables us to tune the electrical and thermoelectric
response of gold molecular junctions. We also show that gold's SS appears in
our simulations only if the 5d orbitals are included explicitly in the valence
shell. To illustrate this behaviour, we discuss in detail Benzenediamine (BDA)
and Benzenedicarbonitrile (BDCN) gold (111) junctions.
