Molecular dynamics simulations have been performed to understand true atomic resolution, which has been observed on the Si(111)-7 X 7 surface by dynamic force microscopy (DFM) in ultra high vacuum (UHV). Stable atomic-scale contrast is reproduced in simulations at constant mean height above a critical tip-sample separation when monitoring the interaction force between tip and sample. Missing or additional adatoms can be recognized in such scans, although they are less well resolved than native adatoms. The resonance frequency shift, as well as arbitrary scans, e.g., at constant force, can be computed from a series of force-distance characteristics. By means of dynamic simulations, we show how energy losses induced by interaction with an oscillating tip can be monitored, and that they occur even in the non-contact range.