Failure of perturbative DFT-Derived STM images of organic molecules on semiconductor surfaces

Tersoff-Hamann and Bardeen perturbation theory derived empty-state STM images of the [4 + 2] intradimer adduct of 1,3-cyclohexadiene (CHD) adsorbed on Si(100)-2 × 1 using a density functional description of the electronic structure appear to capture the π* CdC orbital observed experimentally but ultimately produce images dominated by the CH₂ groups. Neither averages over finite temperature structures nor use of W(110) tips with O or Si adsorbed on the apex yield the correct image. Strong tip-CHD interactions substantially lower the energy of the π* orbital relative to the Fermi energy and change the CHD geometry. In addition to perturbing the CdC bond, it is found that the tip pushes aside the CH₂ groups. The former electronic effect enhances the prominence of the π* orbital, while the latter geometric effect suppresses contributions from the remainder of the CHD.