Fen, If your calculation was done correctly, you must have made some convergence study for the thickness of the slab. Your slab should be thick enough so that the lattice parameter in the center of the slab, and the electronic structure of the center of the slab, are the same with respect to bulk. Once you have found this optimal thickness, then atoms of the center of the slab are fixed to their bulk values and everything else is allowed to relax. You might want to use the slab dipole correction feature which is present in the latest versions of Siesta for correcting the fact that charge transfer between the molecule and the surface could create a spurious dipole in your slab, if you only place the molecule on one side.
Since in the center of the slab one should see, essentially, the bulk electronic structure, it should suffice to find a shift in the energy such that the total DOS due to the atoms in the center of the slab is the same as that of the bulk, that is, that the valence and conduction band edges are aligned as in bulk. Adsorption on surfaces can be tricky because of how much charge transfers penetrate in the upper layers. Usually, if this charge transfer is confined to layers above the middle one, you should be safe. Cheers, Marcos On Thu, Feb 24, 2011 at 2:54 AM, Fen Hong <gm030...@gmail.com> wrote: > And how to define this zero point energy, I donot know how to find this > point for different system. > > >
