[QE-users] -help

[Lorenzo Paulatto]

> 1. Is there any procedure to know about nq values which will be okay to 
> get a phonon dispersion with reliable accuracy.

Not really, usually 8x8x8 is enough for small systems (a couple of 
atoms), with 4x4x4 already being quite decent. For systems with a large 
unit-cell it could be even smaller. What you have to think, is the 
distance between atoms: computing a 4x4x4 grid is equivalent to using a 
4x4x4 supercell, is this large enough to have no interactions between 
periodic copies?

> 2. Is there anything similar thing to do like we do a k-mesh convergence 
> test for scf calculation in order to get a reliable electronic band 
> structure.

Yes, that is the ideal, you do a 2x2x2, then a 4x4x4 then a 6x6x6 and in 
every case you do a dispersion plot with q2r+matdyn, and see how much 
the points which are not in the original grid change. Unfortunately, 
ph.x does not reuse points from previous calculations (i.e. when doing 
the 4x4x4 after 2x2x2 some points are already available) but you can 
skip them by hand with some headscratching if you are so tight on cpu time.

> 3. Is nq and nk grid similar ?

They are both wavevectors of something in a crystal. In one case it is 
the electronic wavefunctions, in the other it is the monochromatic 
perturbation that we usually call "phonon".

hth

> 
> I tried to give all the information I could, I m new to QE so can't 
> explain my doubts better than this, so, please update whatever possible.
> 
> Thanks in advance
> 
> Best,
> Kiran
> 
> On Tue, Nov 3, 2020, 18:22 Lorenzo Paulatto <paulatz at gmail.com 
> <mailto:paulatz at gmail.com>> wrote:
> 
>      > Is there any methodology to check and decide nq-grid points for
>     phonon
>      > dispersion spectra calculations?
> 
>     I'm not sure what calculation you are trying to do.
>     Can you provide a more detailed description?
> 
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> 
> 
>     -- 
>     Lorenzo Paulatto - Paris
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-- 
Lorenzo Paulatto - Paris