[Wien] regarding k-mesh and tolf

[chin Sabsu]

 Dear Respected Xavaier SirThank 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
  
    
  
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