[gpaw-users] problem with the vacuum issue of graphene

[Thomas Olsen]

Dear chi-hsuan

Consider what the band structure would look like if you did not have the 
graphene slab. Then it would just be the energy levels of a box with 
E_n(k) = k^2/2 +n^2/L^2. The spacing of the bands thus depends on the 
box length L. In the high energy regime of an ab initio calculation the 
eigenstates will approach plane waves and you will see something 
similar. Essentially the high energy part of the spectrum approaches the 
vacuum states.

I think most of the quantities we usually calculate will be independent 
of the vacuum spacing used. What do you need the high energy bands for?

/Thomas

On 11/26/2013 10:08 AM, ??? wrote:
> Dear all,
> When I calculate the band structure of graphene.
> I set the distance between two sheets as 15~29 angstrom
> It turns out that the shape of the band structure changed in the high 
> energy region(30~40eV)
> in the gamma-K direction.
>
> I thought the distance of the vacuum would not change the band structure.
> But the result is not so.
> Here's the input which I use to calculate the ground state.
> ======skip the import part
> a=1.42
> c=27
> h = cutoff2gridspacing(100 * Rydberg)
> atoms = Atoms('C2',[                  # Generate graphene AB-stack 
> structure.
>               (1/3.0,1/3.0,0),
>               (2/3.0,2/3.0,0),
>               ],
>               pbc=(1,1,1))
> atoms.set_cell([(sqrt(3)*a/2.0,3/2.0*a,0),
>                 (-sqrt(3)*a/2.0,3/2.0*a,0),
>                 (0.,0.,c)],
>                scale_atoms=True)
>
> calc = GPAW(xc='LDA',
>             kpts=(25,25,1),           # The result should be converged 
> with respect to kpoints.
>             h=h  , # 0.2,
>             basis='dzp',              # Use LCAO basis to get good 
> initialization for unoccupied states.
>             nbands=70,                # The result should also be 
> converged with respect to bands.
>             convergence={'bands':60}, # It's better NOT to converge 
> all bands.
>             eigensolver='cg',         # It's preferable to use 'cg' to 
> calculate unoccupied states.
>             occupations=FermiDirac(0.05),
>             txt='out_gs.txt')
>
> atoms.set_calculator(calc)
> atoms.get_potential_energy()
> calc.write('graphene.gpw','all')
> =========
>
> BR,
> chi-hsuan
>
>
> _______________________________________________
> gpaw-users mailing list
> gpaw-users at listserv.fysik.dtu.dk
> https://listserv.fysik.dtu.dk/mailman/listinfo/gpaw-users

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://listserv.fysik.dtu.dk/pipermail/gpaw-users/attachments/20131128/ad6230e0/attachment.html>