  We have performed the most realistic simulation to date of the operation of a
scanning tunneling microscope. Probe-sample distances from beyond tunneling to
actual surface contact are covered. We simultaneously calculate forces, atomic
displacements, and tunneling currents, allowing quantitative comparison with
experimental values. A distance regime below which the probe becomes unstable
is identified. It is shown that the real distance differs substantially from
previous estimates because of large atomic displacements on the surface and at
the probe-tip.
