Quantum chemical modeling of the scanning tunneling microscope

An approach to the calculation of the tunneling current in STS and STM experiments from the results of quantum chemical atomic cluster calculations, is presented. Essential features of the model are that the final states involved in the expression derived by Tersoff and Hamann, are excited states rather than ionised states, and furthermore these states have to be calculated from multiconfiguration Cl wavefunctions. The method is applied to measurements on the clean and hydrogenated Si(100) surface. Good agreement with measured STS curves is obtained, but only when the dimer geometry is allowed to respond to the biasing voltage to give a zero buckling of the dimer when a positive bias voltage is applied to the surface, and a buckling of 0.41 Å for a negative bias. For the hydrogenated surface the tunneling is found to drastically weaken the dimer bond. Good qualitative agreement is also found for calculated and measured topography curves for an STM tip scanning over the dimer in the dimer plane.