From the other potential intermediates for which adsorption geometries and AFM images have been calculated we can with high confidence rule out structures 20 , 21 , 22 , 23 , and 25 which do not fit to the experimental AFM data . In the simulated AFM images of structures 20 - 23 , at the onset of repulsive interaction ( 4 th column ) , the entire outline of the molecule appears with dark contrast , in contrast to the experiment . In addition , the range of tip - sample distances to simulate AFM images from the onset of repulsive / bright contrast at the highest part to repulsion on the lower laying parts of the molecule is smaller than in the experiment . The simulated contrast of structure 25 is significantly different to any of the measured molecules .