  We report numerical results for two models of vicinal motion. The first, LW,
aims at crystal evaporation when the detachment from steps is slow [Liu and
Weeks, PRB 57, 23 (1998) 14891]. The source of destabilization is
electromigration force acting on the adatoms. The destabilizing part of
equation(s) of the step velocity is linear in the widths of the adjacent
terraces with larger contribution of the terrace behind. This asymmetry is
controlled by a single parameter b. The stabilization part accounts for the
tendency to equidistant spacing dictated by the interstep repulsions. We
construct the second model, LW2, from LW in the same manner as was constructed
Minimal Model 2 (MM2) from another minimal model [B.Ranguelov et al.,
Nanoscience and Nanotechnology 6, (2006) 31] - keeping the 'repulsions term'
from LW and introducing a similar one with opposite sign as 'attractions term'.
For LW we obtain for first time that in the pre-factor of the time scaling of
the number of steps in the bunch N enters only the parameter b. In LW2 we find
the same type of step bunching as in MM2 - the surface slope in the bunch is
constant and not function of N. Further, we obtain the time scaling of N with
exponent ~0.18, found also in experiments, and the minimal distance in the
bunch as function of the model parameters.
