[Pw_forum] << Surface calculations setup >>

[Gabriele Sclauzero]

I. Camps wrote:
> Dear PWscf users,
> 
> I am new to QE and to PWscf.
> 
> I would like to know how to set up surface calculations (I read the 
> on-line manual, FAQs, examples, etc.).

Like in any other 3D-periodic electronic structure code. You use a slab geometry, made of 
bulk layers, which are atomic planes perpendicular to the surface normal.
You have to chose a direction (typically the z axis) on which you "remove" the periodicity 
by adding a vacuum region, while your periodicity in the xy plane depends on the Miller 
indexes of the surface (and on the coverage of adsorbed molecules, if any).

> 
> My questions:
> 
> 1-) Is there a formal definition to a surface (in the same way I define 
> a crystal with Bravais lattice, space group, etc.) or I have to use a 
> bunch of atoms representing the surface?

You have to chose one of the 14 Bravais lattices used for bulk calculations, depending on 
the symmetry of the surface and stack atomic layers along a direction. For instance the 
FCC (100) surface has square symmetry and I would use a tetragonal lattice (ibrav=6), 
while the (110) has rectangular symmetry and I would use orthorhombic lattice. For (111), 
which has hexagonal symmetry, I would an use orthorhombic cell with 3-fold symmetry, since 
the ABC stacking of layers lowers the symmetry.

> 2-) How do I select from the crystal which atoms correspond to a given 
> surface?
> 
> I will be very grateful if you can indicate me any resource in order to 
> learn this.

There is something in the examples:

example03:
     This example shows how to use pw.x to compute the equilibrium
     geometry of a simple molecule, CO, and of an Al (001) slab.
     In the latter case the relaxation is performed in two ways:
     1) using the quasi-Newton BFGS algorithm
     2) using a damped dynamics algorithm.

WorkFct_example:
     This example shows how to use pw.x, pp.x, and average.x to
     compute the work function of a metal using the slab-supercell
     approximation.  This example is of a 4 layer unrelaxed Al(100) slab
     with 5 equivalent layers of vacuum between the surfaces.


regards,

GS



> 
> []s,
> 
> Camps
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| Gabriele Sclauzero, PhD Student                  |
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