[QE-users] Question rgd. q grid for HSE calculations

[Shivesh Sivakumar]

Dear Giuseppe,

Thank you very much for your inputs in this regard.

Best,
Shivesh

On Tue, Feb 2, 2021 at 1:42 AM Giuseppe Mattioli <
giuseppe.mattioli at ism.cnr.it> wrote:

>
> Dear Shivesh Sivakumar
> I have used a 1x1x1 q grid together with HSE or B3LYP in the case of
> innumerable large systems since 2009, with quite satisfactory results.
> Tests made by yourself are the best when you are not sure. You can do
> as I did years ago: start from the unit cell of a very simple system,
> e.g., bulk Si, and try to converge eigenvalues (e.g. the valence band
> maximum at gamma, so that you don't have to refold) by increasing the
> q mesh, and then double the cell size and do the same, and then go up
> until eigenvalues at gamma do not depend anymore on the q grid.
> Moreover, let us suppose that you have thousands of electrons in your
> system. If you badly need exact-exchange to correct delocalization
> errors and you can't use a fully converged q-mesh because it would
> require 1000 GB RAM you have not, then you must choose the best
> trade-off you can afford, and I would go for the 1x1x1 q-mesh. As long
> as you discuss in the methods section of your papers what you do (and
> why), you can do everything sensible.
> HTH
> Giuseppe
>
> Quoting Shivesh Sivakumar <shiveshsivakumar at gmail.com>:
>
> > Hello users,
> >
> > I was just wondering if anyone had any inputs on this: Using a 1x1x1 q
> grid
> > for HSE calculations in the case of 'large non-metallic systems'.
> > I would greatly appreciate any points or resources regarding this.
> >
> > Sincerely,
> > Shivesh Sivakumar
> > Graduate Teaching Assistant
> > Materials Science and Engineering
> > University of Washington-Seattle
> > WA-98105
> > (206) 474-8174
> >
> > On Fri, Jan 29, 2021 at 7:15 PM Shivesh Sivakumar <
> > shiveshsivakumar at gmail.com> wrote:
> >
> >> Hello users,
> >>
> >> I was seeking a clarification in the documentation for calculation of
> the
> >> exact exchange in HSE calculations. Paraphrasing what's mentioned in the
> >> documentation:
> >>
> >> Specifically, in the case of large non-metallic systems, it is
> reasonable
> >> that NQS can be set to 1 (where NQS = nqx1*nqx2*nqx3).
> >>
> >> My qualms are:
> >> 1. I wonder how loose the definition of 'large non-metallic systems' is.
> >> Does it just stipulate a large enough bandgap? (Even though KS-DFT
> >> underestimates it?)
> >> 2. Why does the nature of the system allow us to adopt a very coarse q
> >> grid?
> >>
> >> I would greatly appreciate any inputs on these points.
> >>
> >>
> >> Sincerely,
> >> Shivesh Sivakumar
> >> Graduate Teaching Assistant
> >> Materials Science and Engineering
> >> University of Washington-Seattle
> >> WA-98105
> >> (206) 474-8174
> >> _______________________________________________
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>
>
>
> GIUSEPPE MATTIOLI
> CNR - ISTITUTO DI STRUTTURA DELLA MATERIA
> Via Salaria Km 29,300 - C.P. 10
> I-00015 - Monterotondo Scalo (RM)
> Mob (*preferred*) +39 373 7305625
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> E-mail: <giuseppe.mattioli at ism.cnr.it>
>
> _______________________________________________
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