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

[MKondrin]

Nicola Marzari wrote:
>
>
>
> Dear Manuel,
>
>
> the only explanation to me is that the code didn't find all the
> symmetries that should be there - either because the atomic positions
> were not specified with enough accuracy (so the actual unit cell doesn't
> have those symmetries) or (maybe?) there is something missing in the
> symmetry finder for the rombohedral case. How many operations did the 
> code
> find in the ibrav=0 or the ibrav=4 case? A change in lattie constant 
> (celldm(1))
> wouldn't change the symmetries in your problem.
>
>             nicola
Sorry, but the space group R-3m does not have fractional translations. 
36 symmetry operations are counted in respect to conventional unit cell 
(which is 3 times larger than the primitive one) so I think that QE 
indeed found all symmetry operations.

Best regards,

M. V. Kondrin
>
>
> On 30/05/2015 12:40, MKondrin wrote:
>> Hi!
>>
>> I suppose that forces acting on atoms located on positions fixed by
>> symmetry might indicate that the lattice constants are not optimal one
>> (in fact they depend on pseudopotentials selected). So you should start
>> from 'vc-relax' method and then proceed to 'scf' calculations.
>>
>> Hope this help.
>>
>> Best regards,
>>
>> M. V. Kondrin
>> (High Pressure Physics Institute, RAS)
>>
>> 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
>>>
>>> ------------------------------------------------------------------------ 
>>>
>>>
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