Strange behaviour with spin, but only in supercell

Questions and doubts about the spin-dependent version of Yambo.

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Strange behaviour with spin, but only in supercell

Postby FalkoSchmidt » Tue Aug 21, 2018 1:14 pm

Dear Yambo community,

since I am currently working on a rather large system (polaron at a point defect in a lithium niobate supercell with 80 atoms), I started by calculating the linear response for two test systems, both without point defects and polarons (stoichiometric): The primitive cell with 10 atoms and without spin (system "1" in my notation), and the 2x2x2 repetition with 80 atoms with spin ("29"). Additionally, I used two waypoints: A 10 atom cell with spin ("3") and an 80 atom cell without spin ("18").

Please have a look at the attached figure (plot.pdf): "Switching on" the spin in the small cell ("1"->"3") does not change the result noticeably, as expected. Neither does changing to the supercell ("1"->"18"). Technically, there are changes, but those had to be expected, since I used less k-points. The strange behaviour occurs, when we try to calculate the supercell including spin: The linear response is way too large (factor 4) and its form does not fit too well, either.

Im am not certain how to explain this, since the energies in chapter 02.05 of the file r-IPA_optics_chi are very similar for systems "18" and "29".

Attached you will find all files I used for my calculations. I would be grateful for any suggestions.

Best,
Falko
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Falko Schmidt
Department of Physics
University of Paderborn, Germany
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Re: Strange behaviour with spin, but only in supercell

Postby Davide Sangalli » Tue Aug 21, 2018 4:21 pm

Dear Falko,
your result is indeed strange.

Can I ask you to try the same calculation (just cases 18 and 29) neglecting the non local part of the pseudo in the calculation of the dipoles, to check if the problem comes from there.
The calculation will become significantly faster. You just need to rename the file SAVE/ns.kb_pp_pwscf
Code: Select all
mv SAVE/ns.kb_pp_pwscf SAVE/ns.kb_pp_pwscf_hide

before running the same input files with a different jobstring name.

Best,
D.
Davide Sangalli, PhD
CNR-ISM, Division of Ultrafast Processes in Materials (FLASHit) and MaX Centre
http://www.ism.cnr.it/en/davide-sangalli-cv/
http://www.max-centre.eu/
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Re: Strange behaviour with spin, but only in supercell

Postby andrea.ferretti » Wed Aug 22, 2018 10:09 am

Dear Falko,

I've examined your files and it seems that the failure (in terms of numerical results) of your supercell spin polarized run (#29) is due to a p2y bug that we have recently spotted and fixed.
In particular, here is a description of the bug:

* the bug affects yambo v4.2.0-v4.2.2 interfacing with QE > 6.2.x, if
- QE is not compiled against HDF5 and the QEXSD format is used (both options are the default)
- p2y fragments wfc files over bands (default is only over kpts, unless large systems are treated, as is your case here)
or p2y -b xxx is used.
In practice, if only files of the form SAVE/ns.wf_fragment_X_1 are present, p2y works fine, while if any
SAVE/ns.wf_fragment_X_Y (Y>1) is there, the bug shows up.

* the bug has been fixed in yambo v4.2.3

Hope it helps
Andrea
Andrea Ferretti, PhD
CNR-NANO-S3 and MaX Centre
via Campi 213/A, 41125, Modena, Italy
Tel: +39 059 2055322; Skype: andrea_ferretti
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Re: Strange behaviour with spin, but only in supercell

Postby FalkoSchmidt » Wed Aug 22, 2018 3:19 pm

Dear Davide and Andrea,

upgrading to the newest version of Yambo did indeed solve all of my problems. Thanks a lot for your help!

Best,
Falko
Falko Schmidt
Department of Physics
University of Paderborn, Germany
FalkoSchmidt
 
Posts: 2
Joined: Tue Jul 24, 2018 2:19 pm


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