[Wien] regarding k-mesh and tolf

[Xavier Rocquefelte]

Could you give the chemical formula? It will really help answering your 
question from my side.

Cheers

Xavier


Le 27/01/2018 à 14:34, chin Sabsu a écrit :
> Dear Respected Xavaier Sir
> Thank you very much for detailed explanation.
> My system is not having any d-d interaction (no TM element) so I do 
> not think I should go for  DFT+U, Onsite Hybrid functional and Full 
> hybrid calculations!
>
> With TB-mBJ it is showing a band gap. The VBM is widely dispersed, 
> flat for a wide range, near the VBM. So this is a semiconductor.
>
> As we can not relax the structure using mBJ so there is no way to use 
> it for the phonopy purpose (phonon bands).
>
> Due to lack of good computational facilities, I want to avoid hybrid 
> calculation which will never finish if I go for Phonopy.
>
> What I see it, I use the combination of tolf and -fc such that it 
> gives minimum forces. But I am sure the forces will not be less than 
> 2mRy/Bohr. Again this process I see a contradiction with Peter Sir's 
> statement: forces should be less than 1mRy/bohr for phonon 
> calculations and which I will not be having!!
> How much the ~2-3mRy/bohr forces on the atom affects the phonon band 
> structure?
> can I proceed for phonon bands with the structure having forces ~2-3 
> mRy/Bohr?
>
> Thank you very much!
>
> Chin S.
>
>
>
>
> On Saturday 27 January 2018, 6:31:35 PM IST, Xavier Rocquefelte 
> <xavier.rocquefelte at univ-rennes1.fr> wrote:
>
>
> Dear Chin S.
>
> Your system is metallic from the point of view of DFT which is not a 
> surprise.
>
> In your last calculation, which is the more accurate you simply catch 
> this fact ... In other words in your previous calculations you had 0.1 
> eV gap due to less kpoints and/or less accurate calculations. To 
> really understand what is happening you should draw the DOS and 
> bandstructure to see if you have band crossing at the Fermi level 
> which could lead to occupancy of an expected empty band. If yes, it 
> will have an impact on the chemical bond description and also on the 
> phonons.
>
> Depending on the system you will have to consider methods going beyond 
> DFT, which in WIEN2k could be based on:
>
> - DFT+U
>
> - Onsite Hybrid functional
>
> - Full hybrid
>
> - TB-mBJ potential
>
> If your band gap is based on charge transfer transitions you cannot 
> used DFT+U and onsite-hybrid.
> Thus the less expensive approach will be TB-mBJ potential which will 
> give the band gap but not the forces.
>
> Thus only one option remains which is full hybrid.
>
> If your system is based on d-d transitions, or highly correlated 
> states, then you can used DFT+U or Onsite-hybrid.
>
> Hope it will help you.
>
> Cheers
>
> Xavier
>
>
> Le 27/01/2018 à 12:28, chin Sabsu a écrit :
>> Dear Stefaan Sir
>>
>> Below are my updates:
>>
>> A. For mesh size
>> I got your point and in all different distorted structure (with 
>> different inequivalent atoms) I am having the same number of 
>> electrons while the number of inequivalent atoms are different. NE 
>> varies from 154 to 616(1x1x1 cell) to 4928 (2x2x2 cell).
>> So I will divide xxx numk by 4 and 32 in 1x1x1 and 2x2x2 supercell 
>> structure. Thanks for this excellent recommendation!!!
>> For 2x2x2 supercell in phonopy calculations, we need to go for mpi 
>> jobs for certain cases!!
>>
>> B. Now I have done some tests for my cubic perovskite and below are 
>> my observations:
>>
>> B1. For tolf 5 and -fc1 I for FOR003 = ~4 mRy / bohr   with band gap 
>> ~0.1 eV
>> B2. For tolf 2 and - fc 0.1 forces are almost same. and the band gap 
>> is still ~0.1ev
>>
>> B3. Now to reduce the forces less then 0.1mRy/ bohr , I reduced the 
>> tolf to 0.5 and - fc 0.1 - 0.01. I got forces at the desired accurecy 
>> but the band gap vanished and the system becomes metallic. The 
>> experimentally reported band gap is 1.2eV
>> I used RKMAX 8, numk 2000, vxc 19.
>>
>> What is your suggestion for point number B, why I am getting the 
>> system metallic?
>> Can I use this relaxed structure for Phonopy?
>>
>> Kind regards
>>
>> Chin S.
>>
>>
>> For t olf
>>
>>
>>
>> _______________________________________________
>> Wien mailing list
>> Wien at zeus.theochem.tuwien.ac.at <mailto:Wien at 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 at zeus.theochem.tuwien.ac.at <mailto:Wien at 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 at 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

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://zeus.theochem.tuwien.ac.at/pipermail/wien/attachments/20180127/9e09e114/attachment.html>