[DFTB-Plus-User] Bandgap of GNR

[Gabriele Penazzi]

Dear Kazuma,

the distance between layers in x direction is very large and there's no
interaction between periodic images along x, therefore the band would be
flat. You only need sampling beyond Gamma along direction with
interaction between periodic images, y in your case.

You structure is going to relax if allowed to, therefore the geometry is
not optimized. In particular the C-H bond is shorter than the mio sk set
would deliver.

I used a small broadening factor in the DOS (0.01) to clearly
distinguish the energy levels, or alternatively you can look for the
highest occupied and lowest unoccupied level in the detailed.out. If I
use the default broadening I get a very similar curve to yours, there's
nothing wrong with it.

You're probably trying to decide how large the gap is checking where the
DOS goes to zero, but this is somewhat arbitrary because the DOS is also
calculate given a finite broadening.

If you try to calculate the band structure instead of the DOS you'll see
the gap more clearly.

Best,
Gabriele


On 01/13/2016 11:31 AM, u3129004 at edu.gifu-u.ac.jp wrote:
> Dear Gabriele,
> 
> Thank you very much for your early reply. I have questions.
> 
> *Why I do not need k-points in x direction?
> *I do not know the meaning you said "your structure seemed not optimized
> ".
> 
> In addition, I tried again,but the result is like an attached file. what 
> is the cause that my result do not agree with your result? I also 
> attached the input file.
> 
> Sincerely yours,
> Kazuma 
> 
> ----- Original Message -----
>> Hi Kazuma,
>>
>> you don't need k-points in x direction here. I've run your input with
>> the supercell folding set to 1 in x direction (left 25 in y). I 
> lowered
>> the force tolerance to 1e-3 too (didn't want to wait too long) and I 
> get
>> 1.42 eV (detail of dos around gap attached). Your structure seemed not
>> optimized but this should not be the problem, even if I run the input 
> as
>> it is with the geometry you sent but removing optimization I get
>> something around 1.3 eV. Maybe you do some mistake when evaluating the 
> gap.
>>
>> Best
>> Gabriele
>>
>>
>> On 01/13/2016 10:04 AM, u3129004 at edu.gifu-u.ac.jp wrote:
>>> Dear DFTB+ user,
>>>
>>> I calculated Density of states of graphene nanoribbon (please refer 
> to 
>>> an attached file). But I am doubtful of this result. Because the 
> bandgap 
>>> is a lot different from other calculation results which are 
> calculated 
>>> by Tight Binding method and first principal calculation. I attached 
> my 
>>> input file for this calculation.
>>>
>>> It would be very helpful if you could provide me any advice.
>>>
>>> Thank you in advance,
>>> Kazuma
>>> ---------------------------------------------------
>>> Kazuma KIHIRA
>>> Environmental & Renewable Energy Systems Division
>>> Graduate School of Engineering, Gifu University
>>> E-mail:u3129004 at edu.gifu-u.ac.jp
>>> ---------------------------------------------------
>>>
>>>
>>>
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> 
>>>
>>
>> -- 
>> --
>> Dr. Gabriele Penazzi
>> BCCMS - University of Bremen
>>
>> http://www.bccms.uni-bremen.de/
>> http://sites.google.com/site/gabrielepenazzi/
>> phone: +49 (0) 421 218 62337
>> mobile: +49 (0) 151 19650383
>>  
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-- 
--
Dr. Gabriele Penazzi
BCCMS - University of Bremen

http://www.bccms.uni-bremen.de/
http://sites.google.com/site/gabrielepenazzi/
phone: +49 (0) 421 218 62337
mobile: +49 (0) 151 19650383