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

[Giuseppe Mattioli]

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
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E-mail: <giuseppe.mattioli at ism.cnr.it>