Molecular Orbital Shift of Perylenetetracarboxylic-Dianhydride
  Using low-temperature scanning tunneling microscopy we find that perylenetetracarboxylic-dianhydride on Au(788) exhibits three coexisting adsorption phases. Single-molecule tunneling spectroscopy reveals orbital energies, which differ in the different adsorption phases. Density functional theory calculations associate the experimentally observed submolecular corrugation to the spatial distribution of the second-to-lowest unoccupied molecular orbital. We tentatively attribute the orbital shifts to a varying number of hydrogen bonds. 
J. Kroeger, H. Jensen, R. Berndt, R. Rurali, N. Lorente