[Pw_forum] wrong symmetries for the forces in a supercell (two files enclosed, .in and .out)

[Michael Mehl]

Since pwscf doesn't recognize the (2/3,1/3,1/3) and (1/3,2/3,2/3) 
fractional translations needed to populate the hexagonal cell, it's not 
going to recognize that the Cu atoms are on the (3a) Wyckoff site, or Cr 
on (3b). Given that, the small forces shown here are close enough to 
zero so as to be negligible. They're a full order of magnitude smaller 
than the forces on the oxygen atoms.

If you really want zero force on the Cu and Cr atoms, convert to the 
primitive (rhombohedral) unit cell, which only has four atoms. See 
IBRAV=5.  Of course you'll have to work out the conversion from (a,c) to 
(a',alpha), but that's not too difficult.  See

https://web.archive.org/web/20100916052840/http://cst-www.nrl.navy.mil/lattice/struk/f5_1.html

(To those who are interested: we are obtaining funding to update the 
Crystal Lattice site, including conforming to security guidelines, and 
get it up and available once again, hopefully by the end of summer. No 
promises, I'm afraid.)


On 05/29/2015 10:36 AM, Manuel Pérez Jigato wrote:
>
> hello
>
> I am running an scf calculation for a supercell that is known to have 
> 36 symmetry operations. This supercell (12 atom) contains three 
> primitives of
> the rhombohedral system CuCrO2 (primitive with 4 atom cell), which is 
> known to have 12 symmetry operations. The supercell is a separate
> setting (hexagonal setting) of the rhombohedral space group R-3m (ITA 
> number 166) , which is R-3m:H, and, as such, it is a hexagonal system
>
> By looking at the Wyckoff positions of my supercell atoms, the site 
> symmetries show that all Cu and all Cr atoms are fixed, ie they do not 
> have any degree of freedom,
> whilst all the oxygen atoms have their z coordinate not fixed by symmetry
>
> The reason for the above explanation is that when I run scf/single 
> point energy calculation for the 12 atom supercell I get forces with 
> wrong symmetries, ie
>
>      atom    1 type  1   force =     0.00000000    0.00000000   
>  0.00000000
>      atom    2 type  1   force =     0.00000000    0.00000000   
> -0.00022475
>      atom    3 type  1   force =    -0.00000000   -0.00000000   
>  0.00022475
>      atom    4 type  2   force =    -0.00000000    0.00000000   
>  0.00000000
>      atom    5 type  2   force =     0.00000000    0.00000000   
>  0.00035717
>      atom    6 type  2   force =     0.00000000   -0.00000000   
> -0.00035717
>      atom    7 type  3   force =     0.00000000    0.00000000   
>  0.00241988
>      atom    8 type  3   force =     0.00000000    0.00000000   
> -0.00241988
>      atom    9 type  3   force =    -0.00000000    0.00000000   
>  0.00239382
>      atom   10 type  3   force =     0.00000000    0.00000000   
>  0.00239626
>      atom   11 type  3   force =     0.00000000    0.00000000   
> -0.00239626
>      atom   12 type  3   force =     0.00000000   -0.00000000   
> -0.00239382
>
> the first 3 lines correspond to Cu, the next 3 lines are Cr, and the 
> last 6 lines are oxygen
>
> According to the site symmetries, only the z components of the force 
> on the oxygen atoms (bottom six lines)
> should be different from zero, but, the output shows that there are z 
> components of the force on Cu and Cr (not all of them) different from 
> zero;
> as far as I can see, they should be zero
>
> This problem appears when I run scf under ibrav 0 and also when I use 
> ibrav 4.
> After seeing the non-zero forces on Cu and Cr, I have decided to 
> generate the input geometry not by hand, but
> by means of cif2cell, which gives the ibrav 0 option. Still, the 
> problem of non-zero forces persists.
> According to the author of cif2cell, the input geometry generated by 
> cif2cell is correct, as well as the CIF file I started from./(See 
> attached file: otra.in)//(See attached file: otra.out)/
> He suggested that I write to this forum in order to find out about the 
> problem, since he agrees with me on the point that
> all force components should vanish for all copper and chromium atoms, 
> and that there should be non-zero forces just on all oxygens (z component)
>
> will you please help with this? probably some mistake from my side...
>
> thanks
>
> Manuel
> PS in order to make sure the FFT grid does not break any symmetries, I 
> run the example with high cutoff, but the same thing happens
> The k-point set does not break symmetries, since it is Cunningham, ie 
> it contains the gamma-point (odd grid-point number for each of the 
> three lattice directions)
>
> 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.perez at list.lu
>
>
> _______________________________________________
> Pw_forum mailing list
> Pw_forum at pwscf.org
> http://pwscf.org/mailman/listinfo/pw_forum


-- 
Michael J. Mehl
Head, Center for Computational Materials Science
Naval Research Laboratory Code 6390
Washington DC
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