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

[Thomas Olsen]

Dear Chi-hsuan

You should look at EELS spectrum and see if it converged with respect to 
vacuum and with respect to unoccupied bands. Although the high-lying 
bands may contribute to the screening, you should not worry too much 
about the exact structure of these bands (dependence of vacuum). They do 
not explicitly contribute to the EELS spectrum, but merely act as a 
basis for the perturbation.

/Thomas

On 11/28/2013 09:00 AM, 謝其軒 wrote:
> Thomas,
> I want to get the resulting electric energy loss function(EELS) of
> graphene. I'm not sure the bands I used are enough or not.
>
> BR,
> chi-hsuan
>
> 2013/11/28, Thomas Olsen <tolsen at fysik.dtu.dk>:
>> 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
>>>
>>>
>>> _______________________________________________
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>>> gpaw-users at listserv.fysik.dtu.dk
>>> https://listserv.fysik.dtu.dk/mailman/listinfo/gpaw-users
>>