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 <shiveshsivaku...@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 <
shiveshsivaku...@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.matti...@ism.cnr.it>
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