  We propose a method to directly couple molecular dynamics, finite element
method and particle-in-cell techniques to simulate metal surface response to
high electric fields. We use this method to simulate the evolution of a field
emitting tip under thermal runaway by fully including the 3D space-charge
effects. We also present a comparison of the runaway process between the two
tip geometries of different widths. The results show with high statistical
significance that in case of sufficiently narrow field emitters, the thermal
runaway occurs in cycles where intensive neutral evaporation alternates with
the cooling periods. The comparison with previous works shows, that the
evaporation rate in the regime of intensive evaporation is sufficient to ignite
a plasma arc above the simulated field emitters.
