[Wien] Surface Layer Optimization

[Chiung-Yuan Lin]

Dear all,
  I have a question about slab (surface) calculation. I first do a slab
with a its bulk lattice constant (even between the surface layer and
the one next to it).
  Of course, this is an approximation by ignoring the shift of the
surface layer from its bulk position. Then I try to do structural
optimization by shifting the surface layer away from its bulk
position. Do I use the same volume of the unit cell the (then
vacuum spacing changed) or the same vacuum spacing (then
the volume of the unit cell changed)? Do I read the total energy
in case.scf to determine the optimized surface layer position,
or I need a different definition of the total energy for optimization?

Thank you
Chiung-Yuan