Nonperturbative evaluation of STM tunneling probabilities fromab initiocalculations

We present a method for calculating the rate of electron transfer across the tunnel junction in a scanning tunneling microscope without any perturbation expansion in the tip-sample coupling. The method may be readily combined with separate ab initio electronic-structure calculations for the tip and the sample. This involves replacing the asymptotic scattering states by localized initial and final states on either sides of the tunnel junction. We present examples of applications to ${\mathrm{C}}_2{\mathrm{H}}_4$ molecules adsorbed on the Si(001) surface.