[Pw_forum] Fw: SV: SV: SV: cif2cell enquiry TO MKONDRIN ( one file enclosed )

[Manuel Pérez Jigato]

hi mkondrin

the email below, from the author of cif2cell, explains the reasons for having
zero forces for all copper and chromium atoms on the basis of the CIF file site
symmetries; please find enclosed the cif file

regards

Manuel
PS the expected forces, qualitatively speaking are shown below (first 6 lines are oxygen, next three lines are Cr, last 3 lines are copper atoms), from a low cutoff, small number of k-points abinit calculation
(in the espresso calculation some copper and chromium atoms are fixed, other copper and chromium atoms move, which means some of the copper-copper, chromium-copper and chromium-chromium distances vary, and they should all be fixed)
 
cartesian forces (eV/Angstrom) at end:
    1      0.00000000000000     0.00000000000000    33.52744537217701
    2      0.00000000000000     0.00000000000000    33.52744537217701
    3      0.00000000000000     0.00000000000000    33.52744537217700
    4      0.00000000000000     0.00000000000000   -33.52744537217701
    5      0.00000000000000     0.00000000000000   -33.52744537217701
    6      0.00000000000000     0.00000000000000   -33.52744537217700
    7      0.00000000000000     0.00000000000000     0.00000000000000
    8      0.00000000000000     0.00000000000000     0.00000000000000
    9      0.00000000000000     0.00000000000000     0.00000000000000
   10      0.00000000000000     0.00000000000000     0.00000000000000
   11      0.00000000000000     0.00000000000000     0.00000000000000
   12      0.00000000000000     0.00000000000000     0.00000000000000

Dr Manuel Pérez Jigato, Chargé de Recherche
Luxembourg Institute of Science and Technology (LIST)
Materials Research and Technology (MRT)
41 rue du Brill
L-4422 BELVAUX
Grand-Duché de Luxembourg
Tel (+352) 47 02 61 - 584
Fax (+352) 47 02 64
e-mail manuel.pe...@list.lu

-----Forwarded by Manuel Pérez Jigato/LIST on 05/30/2015 06:35PM -----

To: Manuel Pérez Jigato <manuel.pe...@list.lu>
From: Björkman Torbjörn <torbjorn.bjork...@aalto.fi>
Date: 05/29/2015 02:04PM
Subject: SV: SV: SV: cif2cell enquiry

Well, whether or not it is explicitly used for symmetrization, the symmetry is of course still there. So if by symmetry the forces should be zero, then they should be zero. In your case I can see from the CIF that the forces should indeed be zero, since they are 3a and 3b Wyckoff positions, which have no free parameters. If they are not, then there is some form of numerical problem, how bad it is of course depends on how large the forces are. The manual mentioned that the fourier mesh might not having the symmetry of the lattice in all cases, it may be that it is possible to get that in order by selecting them as multiples of 3 for a trigonal lattice. That's all I can say, I don't know anything specific about QE.

Cheers,
Torbjörn

---
Torbjörn Björkman, PhD
COMP, Aalto University School of Science
Espoo, Finland

---

Från: Manuel Pérez Jigato [manuel.pe...@list.lu]
Skickat: den 29 maj 2015 14:42
Till: Björkman Torbjörn
Ämne: Re: SV: SV: cif2cell enquiry

thanks a lot Bjorkman, it is very kind of you
my only worry would be the possible implications for the forces
I will ask to the pwscf mailing list
regards
Manuel

Björkman Torbjörn ---29/05/2015 12:08:34---Dear Manuel, I can find nothing that looks wrong in the cif2cell output, but a quick look in the "Tr

From: Björkman Torbjörn <torbjorn.bjork...@aalto.fi>
To: Manuel Pérez Jigato <manuel.pe...@list.lu>,
Date: 29/05/2015 12:08
Subject: SV: SV: cif2cell enquiry

---

Dear Manuel,

I can find nothing that looks wrong in the cif2cell output, but a quick look in the "Troubleshooting" section of the QE manual seems to provide the most likely answer:

pw.x does not find all the symmetries you expected

...

• a fractional translation, without rotation, is a symmetry operation of the system. This means that the cell is actually a supercell. In this case, all symmetry operations containing fractional translations are disabled. The reason is that in this rather exotic case there is no simple way to select those symmetry operations forming a true group, in the mathematical sense of the term.

What they say is that QE in cases like these will just say "this is a supercell" and throw the fractional translations away, which gives you 12 symmetries instead of 36, just as it should. I have no idea if you can bypass this behaviour in QE though, better ask on their mailing list.

Hope this helps,
Torbjörn

---
Torbjörn Björkman, PhD
COMP, Aalto University School of Science
Espoo, Finland

---

Från: Manuel Pérez Jigato [manuel.pe...@list.lu]
Skickat: den 29 maj 2015 12:37
Till: Björkman Torbjörn
Ämne: Re: SV: cif2cell enquiry

Dear Bjorkman

thanks for your prompt reply
please find enclosed the files(See attached file: esta.cif)(See attached file: QE_NOREDUCE_esta.in)(See attached file: second.in)(See attached file: second.out)(See attached file: output_noreduce.txt)

Manuel
PS the different between the two .in files is that one of them is the direct output from cif2cell, whilst the other is produced by
adding a few options by hand, before running quantum-espresso (whose output you can see as well)

esta.cif
Description: Binary data

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