[Wien] regarding k-mesh and tolf

[Xavier Rocquefelte]

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 tolf
>
>
>
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