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

[Manuel Pérez Jigato]

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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