[QE-users] -help

[Kiran Yadav]

Dear Lorenzo,
Thankyou very much for providing the great help and information. Hopefully
it will save lots of my time.


Cheers!
Kiran

On Tue, Nov 3, 2020, 19:16 Lorenzo Paulatto <paulatz at gmail.com> wrote:

> > 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?
> >
> >     Do not forget the second law of question-answer conservation:
> >     «In a not-being-paid-for frame of reference,
> >     the amount of time spent answering a question
> >     will never exceed the amount of time spent asking it»
> >
> >
> >
> >     --
> >     Lorenzo Paulatto - Paris
> >     _______________________________________________
> >     Quantum ESPRESSO is supported by MaX (www.max-centre.eu
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> >
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>
> --
> Lorenzo Paulatto - Paris
> _______________________________________________
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