supercell

[sara]

Hi everyone. I have a question. I have a problem with finding atomic
 positions  supercells of the  anatase and rutile  (1 0 1) surface. Can
 I use Nano lab software to find out atomic positions .As I am a new user of ATK , it is confusing me! How can I create a super cell?

[zh]

The slab model by using a supercell is quite common in the computational surface science. You can find too many papers on the surface properties using the slab model. Mostly in the computational details of such paper, the authors may list the procedure of building a supercell for the slab model of a surface.  Strongly suggest you to first do literature survey by yourself.

1. First optimize the lattice constants and atomic coordinates of the bulk phase of a material.
2. Based on the optimized structure of bulk phase in the 1st step, use the "Cleave surface" tool in VNL to cleave a slab with proper atomic layers for of  a surface.
3. In order to include a vacuum layer, increase the lattice constant for the direction perpendicular to the surface
4. Do a geometry optimization with a constraint that the coordinates of atoms in the middle two or three layers are fixed and other atoms are allowed to relax.

[sara]

thanks for your answer

[Jin You Lu]

Hi zh
for the procedure 4 .....
->Do a geometry optimization with a constraint that the coordinates of atoms in the middle two or three layers are fixed and other atoms are allowed to relax.

However, in the tutorial CO/Pd(100), when build Pd(100) surface and relax it.
the constraint is to fix the bottom two layers of the Pd(100) surface not the middle two
http://docs.quantumwise.com/tutorials/geometry_optimization/geometry_optimization.html#build-the-pd-100-surface-and-relax-it
Could you explain this constraint a little bit?
Thanks

[Ulrik G. Vej-Hansen]

It depends on what you want to model. In the tutorial you link to, the purpose is to have one side of the slab model a real surface, and the rest is supposed to model bulk. In this case it is correct to do as written in the tutorial and constrain the bottom two layers.

In other cases you might want to use both sides of the slab as models for a surface, and in this case it could make sense to constrain the layers in the middle, but in general this would probably require a much thicker slab.