  The simulations of field-evaporation processes for silicon atoms on various
Si(001) surfaces are implemented using the first-principles calculations based
on the real-space finite-difference method. We find that the atoms which locate
on atomically flat Si(001) surfaces and at step edges are easily removed by
applying external electric field, and the threshold value of the external
electric field for evaporation of atoms on atomically flat Si(001) surfaces,
which is predicted between 3.0 and 3.5 V/\AA, is in agreement with the
experimental data of 3.8 V/\AA. In this situation, the local field around an
evaporating atom does not play a crucial role. This result is instead
interpreted in terms of the bond strength between an evaporating atom and
surface.
