Figure 2. Generality of the bistable behaviour for benzene derivatives.

(a) Adsorption energy for benzene (C₆H₆) adsorbed on the Pt(111) surface at the most stable bridge-30° site as a function of distance d from the surface. The grey intervals indicate the experimental binding distance based on a low-energy electron diffraction (LEED) analysis45 and adsorption energies determined from microcalorimetry measurements36. (b) The binding energy curves for hexafluorobenzene (C₆F₆), hexachlorobenzene (C₆Cl₆) and 1,2,4,5-tetrachlorobenzene (C₆H₂Cl₄) on Pt(111). (c) Two additional curves for tetrachloropyrazine (C₄N₂Cl₄) and pyrazine (C₄N₂H₄) on Pt(111). The inset shows the structure of the transition state for the C₄N₂Cl₄ molecule. (d) Comparison of the binding energy curves from the PBE+vdW^surf and optB88-vdW methods for the C₄N₂Cl₄/Pt(111) system. For each structure, we fixed the z coordinates of the carbon backbone and the Pt atoms in the bottom four layers of the six-layer surface model. The adsorption height d corresponds to the perpendicular C–Pt distance, which is evaluated relative to the position of the unrelaxed top metal layer.