  The simulations of field-evaporation processes for surface atoms on W(011)
and Mo(011) surfaces are implemented using first-principles calculations based
on the real-space finite-difference method. The threshold values of the
external electric field for evaporation of the surface atoms, which are $\sim$
6 V/\AA 2mm for tungsten and $\sim$ 5 V/\AA 2mm for molybdenum, are in
agreement with the experimental results. Whereas field evaporation has been
believed to occur as a result of significant local-field enhancement around the
evaporating atoms, in this study, the enhancement is not observed around the
atoms but above them and the strength of the local field is much smaller than
that expected on the basis of the classical model.
