[DFTB-Plus-User] Bandgap of GNR

[Gabriele Penazzi]

On 01/13/2016 01:29 PM, u3129004 at edu.gifu-u.ac.jp wrote:
> Dear Gabriele,
> 
> Thank you very much for your advice and teaching. 
> Thank to you, using small boadening factor I could get the same result 
> with yours. I would like to ask you additional questions:
> 
> *How should I decide whether structure is optimized or not? 
> *And in order to optimize this geometry, how should I change the 
> parameter?

Dear Kuzuma,

your input file contains the optimization section already (Driver =
ConjugateGradient). See the manual, the Driver takes care of optimization.

Your system is not optimized if atomic forces are too large.

If you run an optimization the maximum force is given as output, and you
can set a tolerance for the minimum force you what to achieve.

However, if you are not familiar with all these concepts, I suggest you
to ask your supervisor a book on QM or DFT modelling. Many concepts will
apply to DFTB too and it will be definitely more helpful than my
explanations.

Best,
Gabriele

> 
> Sincerely yours,
> Kazuma
> 
> ----- Original Message -----
>> 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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> 
>>>
>>>>
>>>>
>>>>
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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
>> _______________________________________________
>> DFTB-Plus-User mailing list
>> DFTB-Plus-User at mailman.zfn.uni-bremen.de
>> https://mailman.zfn.uni-bremen.de/cgi-bin/mailman/listinfo/dftb-plus-user
> 
>>
> 
> 
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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