The forces between two single molecules brought into contact, and their connection with charge transport through the molecular junction, are studied here using non contact AFM, STM, and density functional theory simulations. A carbon monoxide molecule approaching an acetylene molecule (C_{2}H_{2}) initially feels weak attractive electrostatic forces, partly arising from charge reorganization in the presence of molecular . We find that the molecular contact is chemically passive, and protects the electron tunneling barrier from collapsing, even in the limit of repulsive forces. However, we find subtle conductance and force variations at different contacting sites along the C_{2}H_{2} molecule attributed to a weak overlap of their respective frontier orbitals.

Centro de Fisica de Materiales CSIC UPV EHU 20018 Donostia San Sebastian Spain

CIC nanoGUNE 20018 Donostia San Sebastian Spain

Ikerbasque Basque Foundation for Science 48011 Bilbao Spain

Institut für Experimentalphysik Freie Universität Berlin 14195 Berlin Germany

Institute of Physics of the Academy of Sciences of the Czech Republic 162 00 Prague Czech Republic

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Mapping the electrostatic force field of single molecules from high-resolution scanning probe images