  High resolution Atomic Force Microscopy (AFM) and Scanning Tunnelling
Microscopy (STM) imaging with functionalized tips is well established, but a
detailed understanding of the imaging mechanism is still missing. We present a
numerical STM/AFM model, which takes into account the relaxation of the probe
due to the tip-sample interaction. We demonstrate that the model is able to
reproduce very well not only the experimental intra- and intermolecular
contrasts, but also their evolution upon tip approach. At close distances, the
simulations unveil a significant probe particle relaxation towards local minima
of the interaction potential. This effect is responsible for the sharp
sub-molecular resolution observed in AFM/STM experiments. In addition, we
demonstrate that sharp apparent intermolecular bonds should not be interpreted
as true hydrogen bonds, in the sense of representing areas of increased
electron density. Instead they represent the ridge between two minima of the
potential energy landscape due to neighbouring atoms.
