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We employ normal-incidence x-ray standing wave and temperature programed desorption spectroscopy to derive the adsorption geometry and energetics of the prototypical molecular switch azobenzene at Ag(111). This allows us to assess the accuracy of semiempirical correction schemes as a computationally efficient means to overcome the deficiency of semilocal density-functional theory with respect to long-range van der Waals (vdW) interactions. The obtained agreement underscores the significant improvement provided by the account of vdW interactions, with remaining differences mainly attributed to the neglect of electronic screening at the metallic surface.

A combined study of scanning tunneling microscopy ͑STM͒ and density functional theory ͑DFT͒ reveals that 3,4,9,10-perylene-tetracarboxylic-dianhydride ͑PTCDA͒ adsorbs on Ag͑111͒ at bridge sites in two nonequivalent orientations, one nearly aligned with the ͓101͔ substrate axis and the other 18°misaligned. Site-specific spectroscopy reveals that molecules in the two configurations exhibit subtle differences in their electronic structure. DFT-based STM simulations trace these back to the influence of distinct local adsorption geometries on the chemical molecule-substrate and molecule-molecule interactions.

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