Re: [Wien] Different Plasma Frequency (PF) for bulk and supercell Na

[Dr. K. C. Bhamu]

Dear sajjad

I am also a beginner but I must suggest you to choose 18*18*3 k points for
1*1*6 supercell in comparison with bulk.
As volume in real space is inversely proportional to volume of reciprocal
space. Larger the volume in real space lower the volume in reciprocal
space. So while computing properties of 1*1*6 structure you are minimizing
your volume in z direction and hence need less k-points.

I appreciate any comment on this from DFT experts.


Sincerely
Bhamu

I have computed PF for bulk Na and its value is  *5.943 eV* with k-mesh
> *18*18*18*.
>
> Then I constructed a supercell 1*1*6
>

...>>> use 18*18*3 k points for 1*1*6 supercell.

and run the calculation with different k-meshes and obtained PF values. The
> details are
>
> *K-meshPF (x/z components in eV)*
>
> *24*24*24  5.7097   /   5.8192*
>
>
> *36*36*24  5.7614   /   5.8098  *
>
>
> *50*50*10  5.9358   /   5.6280 *
>
> *50*50*30  5.9365   /   5.8079*
>
> My question is why PF is very sensitive to k-mesh selection? and which
> value of PF is required for correct dielectric constant calculation? Do I
> need *PF = 5.943 eV* for the supercell and then, definitely, need more
> test calculation for right k-mesh selection?
>
> --
> Kind Regards
> Muhammad Sajjad
> Post Doctoral Fellow
> KAUST, KSA.
>
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>
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Re: [Wien] Different Plasma Frequency (PF) for bulk and supercell Na

[Lyudmila Dobysheva]

13.10.2015 09:42, Muhammad Sajjad wrote:

I have computed PF for bulk Na and its value is *5.943 eV* with k-mesh
*18*18*18*.
Then I constructed a supercell 1*1*6:
*K-meshPF (x/z components in eV)*
*24*24*24  5.7097   /   5.8192*
*36*36*24  5.7614   /   5.8098
*50*50*10  5.9358   /   5.6280
*50*50*30  5.9365   /   5.8079*
My question is why PF is very sensitive to k-mesh selection?


I do not know about PF, but general principles are that the larger is a 
supercell the less number of points you need. So, the best described 
direction is z, it corresponds to 180 points in bulk (30x6). Let it be 
that you have reached a convergence in it in supercell, this means that 
you didn't obtain it in a bulk calculation. If you want a comparable 
number of k-points you should have more points in bulk. Maybe 24 in z 
direction is already enough. So, IMHO, try up to 120x120x120 for bulk, 
at least, and further 180x180x180. And for supercell it means 180x180x30

and which
value of PF is required for correct dielectric constant calculation? Do
I need *PF = 5.943 eV* for the supercell


In terms of accuracy, this is the worst.

Best wishes
  Lyudmila Dobysheva
--
Phys.-Techn. Institute of Ural Br. of Russian Ac. of Sci.
426001 Izhevsk, ul.Kirova 132
RUSSIA
--
Tel.:7(3412) 432045(office), 722529(Fax)
E-mail: l...@ftiudm.ru, lyuk...@mail.ru (office)
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Re: [Wien] Different Plasma Frequency (PF) for bulk and supercell Na

[Lyudmila Dobysheva]

13.10.2015 12:58, Muhammad Sajjad wrote:

Should plasma frequency be same for bulk and
supercell?


I do not know about PF, but I'd say yes.


In terms of accuracy, this is the worst.

but why?


Because 30 in z direction in supercell means 180 in usual cell, and you 
have only 18.



  Lyudmila Dobysheva
--
Phys.-Techn. Institute of Ural Br. of Russian Ac. of Sci.
426001 Izhevsk, ul.Kirova 132
RUSSIA
--
Tel.:7(3412) 432045(office), 722529(Fax)
E-mail: l...@ftiudm.ru, lyuk...@mail.ru (office)
lyuk...@gmail.com (home)
Skype:  lyuka17 (home), lyuka18 (office)
http://ftiudm.ru/content/view/25/103/lang,english/
--
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Re: [Wien] Different Plasma Frequency (PF) for bulk and supercell Na

[Muhammad Sajjad]

Dear  Lyudmila and BHamu
Thank you for your suggestions. I have already used 18*18*3 for supercell
and 18*18*18, but found difference in plasma frequency for supercell and
bulk.  Should plasma frequency be same for bulk and supercell? or I should
use the new plasma frequency obtained from super cell calculation?

@Lyudmila
| In terms of accuracy, this is the worst.
but why?



On Tue, Oct 13, 2015 at 10:55 AM, Dr. K. C. Bhamu 
wrote:

>
>
> Dear sajjad
>
> I am also a beginner but I must suggest you to choose 18*18*3 k points for
> 1*1*6 supercell in comparison with bulk.
> As volume in real space is inversely proportional to volume of reciprocal
> space. Larger the volume in real space lower the volume in reciprocal
> space. So while computing properties of 1*1*6 structure you are minimizing
> your volume in z direction and hence need less k-points.
>
> I appreciate any comment on this from DFT experts.
>
>
> Sincerely
> Bhamu
>
> I have computed PF for bulk Na and its value is  *5.943 eV* with k-mesh
>> *18*18*18*.
>>
>> Then I constructed a supercell 1*1*6
>>
>
> ...>>> use 18*18*3 k points for 1*1*6 supercell.
>
> and run the calculation with different k-meshes and obtained PF values.
>> The details are
>>
>> *K-meshPF (x/z components in eV)*
>>
>> *24*24*24  5.7097   /   5.8192*
>>
>>
>> *36*36*24  5.7614   /   5.8098  *
>>
>>
>> *50*50*10  5.9358   /   5.6280 *
>>
>> *50*50*30  5.9365   /   5.8079*
>>
>> My question is why PF is very sensitive to k-mesh selection? and which
>> value of PF is required for correct dielectric constant calculation? Do I
>> need *PF = 5.943 eV* for the supercell and then, definitely, need more
>> test calculation for right k-mesh selection?
>>
>> --
>> Kind Regards
>> Muhammad Sajjad
>> Post Doctoral Fellow
>> KAUST, KSA.
>>
>> ___
>> Wien mailing list
>> Wien@zeus.theochem.tuwien.ac.at
>> http://zeus.theochem.tuwien.ac.at/mailman/listinfo/wien
>> SEARCH the MAILING-LIST at:
>> http://www.mail-archive.com/wien@zeus.theochem.tuwien.ac.at/index.html
>>
>>
>
> ___
> Wien mailing list
> Wien@zeus.theochem.tuwien.ac.at
> http://zeus.theochem.tuwien.ac.at/mailman/listinfo/wien
> SEARCH the MAILING-LIST at:
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>
>


-- 
Kind Regards
Muhammad Sajjad
Post Doctoral Fellow
KAUST, KSA.
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