Scanning tunneling microscopy calculations of a biphenyl molecule adsorbed on a silicon (100) surface allow disentangling the molecular adsorption's configuration as well as the electronic states whose excitation leads to the bistability response observed in recent experiments. The molecular configuration corresponds to one chemisorbed phenyl ring in a butterfly configuration. There is dissociation between the second ring and one hydrogen atom that passivates one Si dangling bond. The molecular motion is triggered by accumulation of charges in the resulting covalent Si-C bond formed between this latter ring and the surface.
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