[QE-users] Is there any problem if I use less ecutwfc and ecutrho than the suggested value in the pp file?

[Andrii Shyichuk]

Dear Endale, 

Using values smaller than recommended might result in errors going
unpredictably high. 
In your case, using e.g. 40/400 instead of 45/455 could happen to be
fine (or not), but 30/300 is most likely bad. 

In order to check it, try a few calculations with different cutoffs. As
the cutoff goes up, band gap should approach some value.
As the cutoff goes down, the point of rapid change in the values is
where things get bad. 
Same goes for DOS, if it starts to change at lower cutoffs - bad. 

Another thing to check is energy-volume curve (equation of state, EOS),
which should be smooth. 
If it's not - the basis is probably too low.
And, it's minimum should converge with the increasing cutoffs. 

I recommend looking into GBRV pseudopotentials (optimized for about 40
Ry cutoff) http://dx.doi.org/10.1016/j.commatsci.2013.08.053. 

Finally, check your basis on some known materials that contain the same
elements as your material of interest. 

Best regards.
Andrii Shyichuk

W dniu 2020-10-20 02:10, ENDALE ABEBE napisał: 

> Dear Expertise 
> 
> Let me say I am calculating the energy bandgap of a semiconductor, which doesn't have reported value in literature. 
> What is the parameter that I need to check to say the ecutwfc/ecutrho is sufficient and the value ot energy bandgap is correct? 
> 
> Thank you for your support 
> 
> With Best Regards 
> 
> On Tue, Oct 13, 2020 at 1:06 AM Paolo Giannozzi <p.giannozzi at gmail.com> wrote: 
> 
> As long as your cutoffs are sufficiently good to get the desired accuracy on the targeted properties, there is no problem. The cutoffs suggested in the PP file are on the safe side and thus quite high, if I remember correctly. 
> 
> Paolo 
> 
> On Mon, Oct 12, 2020 at 2:56 PM ENDALE ABEBE <endtse at gmail.com> wrote: 
> 
> Dear Expertise 
> 
> The suggested values of the ecutrho and ecutwfc are making my calculation expensive. For instance, in the pseudo potential file of Bi, Bi.pbe-dn-kjpaw_psl.1.0.0.UPF [1], these are: 
> 
> Suggested minimum cutoff for wavefunctions:  45. Ry
> Suggested minimum cutoff for charge density: 455. Ry
> Is there any problem if I use less values than these as much as the total energy of the system converges? 
> 
> Thank you in advance for your response. 
> 
> With Regards 
> 
> -- 
> 
> Endale Abebe 
> Program coordinator and Lecturer 
> Faculty of Materials Science and Engineering 
> Jimma Institute of Technology 
> Jimma University
> 
> P.O.Box 378, Jimma, Ethiopia 
> Mobile: +251921381598 _______________________________________________
> Quantum ESPRESSO is supported by MaX (www.max-centre.eu [2])
> users mailing list users at lists.quantum-espresso.org
> https://lists.quantum-espresso.org/mailman/listinfo/users 
> 
> -- 
> 
> Paolo Giannozzi, Dip. Scienze Matematiche Informatiche e Fisiche,
> Univ. Udine, via delle Scienze 208, 33100 Udine, Italy
> Phone +39-0432-558216, fax +39-0432-558222
> 
> _______________________________________________
> Quantum ESPRESSO is supported by MaX (www.max-centre.eu [2])
> users mailing list users at lists.quantum-espresso.org
> https://lists.quantum-espresso.org/mailman/listinfo/users

-- 

Endale Abebe 
Program coordinator and Lecturer 
Faculty of Materials Science and Engineering 
Jimma Institute of Technology 
Jimma University

P.O.Box 378, Jimma, Ethiopia 
Mobile: +251921381598 

_______________________________________________
Quantum ESPRESSO is supported by MaX (www.max-centre.eu [2])
users mailing list users at lists.quantum-espresso.org
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Links:
------
[1]
https://www.quantum-espresso.org/upf_files/Bi.pbe-dn-kjpaw_psl.1.0.0.UPF
[2] http://www.max-centre.eu
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