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<div class="moz-cite-prefix">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.<br>
<br>
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<br>
<br>
<a class="moz-txt-link-freetext" href="https://web.archive.org/web/20100916052840/http://cst-www.nrl.navy.mil/lattice/struk/f5_1.html">https://web.archive.org/web/20100916052840/http://cst-www.nrl.navy.mil/lattice/struk/f5_1.html</a><br>
<br>
(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.)<br>
<br>
<br>
On 05/29/2015 10:36 AM, Manuel Pérez Jigato wrote:<br>
</div>
<blockquote
cite="mid:OF345E457D.9D323DE7-ONC1257E54.004B3F9B-C1257E54.0050522C@list.lu"
type="cite">
<p><font face="sans-serif" size="2">hello</font><br>
<br>
<font face="sans-serif" size="2">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</font><br>
<font face="sans-serif" size="2">the rhombohedral system CuCrO2
(primitive with 4 atom cell), which is known to have 12
symmetry operations. The supercell is a separate</font><br>
<font face="sans-serif" size="2">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</font><br>
<br>
<font face="sans-serif" size="2">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,</font><br>
<font face="sans-serif" size="2">whilst all the oxygen atoms
have their z coordinate not fixed by symmetry</font><br>
<br>
<font face="sans-serif" size="2">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</font><br>
<br>
<font face="sans-serif" size="2"> atom 1 type 1 force
= 0.00000000 0.00000000 0.00000000</font><br>
<font face="sans-serif" size="2"> atom 2 type 1 force
= 0.00000000 0.00000000 -0.00022475</font><br>
<font face="sans-serif" size="2"> atom 3 type 1 force
= -0.00000000 -0.00000000 0.00022475</font><br>
<font face="sans-serif" size="2"> atom 4 type 2 force
= -0.00000000 0.00000000 0.00000000</font><br>
<font face="sans-serif" size="2"> atom 5 type 2 force
= 0.00000000 0.00000000 0.00035717</font><br>
<font face="sans-serif" size="2"> atom 6 type 2 force
= 0.00000000 -0.00000000 -0.00035717</font><br>
<font face="sans-serif" size="2"> atom 7 type 3 force
= 0.00000000 0.00000000 0.00241988</font><br>
<font face="sans-serif" size="2"> atom 8 type 3 force
= 0.00000000 0.00000000 -0.00241988</font><br>
<font face="sans-serif" size="2"> atom 9 type 3 force
= -0.00000000 0.00000000 0.00239382</font><br>
<font face="sans-serif" size="2"> atom 10 type 3 force
= 0.00000000 0.00000000 0.00239626</font><br>
<font face="sans-serif" size="2"> atom 11 type 3 force
= 0.00000000 0.00000000 -0.00239626</font><br>
<font face="sans-serif" size="2"> atom 12 type 3 force
= 0.00000000 -0.00000000 -0.00239382</font><br>
<br>
<font face="sans-serif" size="2">the first 3 lines correspond to
Cu, the next 3 lines are Cr, and the last 6 lines are oxygen</font><br>
<br>
<font face="sans-serif" size="2">According to the site
symmetries, only the z components of the force on the oxygen
atoms (bottom six lines)</font><br>
<font face="sans-serif" size="2">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;</font><br>
<font face="sans-serif" size="2">as far as I can see, they
should be zero</font><br>
<br>
<font face="sans-serif" size="2">This problem appears when I run
scf under ibrav 0 and also when I use ibrav 4.</font><br>
<font face="sans-serif" size="2">After seeing the non-zero
forces on Cu and Cr, I have decided to generate the input
geometry not by hand, but</font><br>
<font face="sans-serif" size="2">by means of cif2cell, which
gives the ibrav 0 option. Still, the problem of non-zero
forces persists.</font><br>
<font face="sans-serif" size="2">According to the author of
cif2cell, the input geometry generated by cif2cell is correct,
as well as the CIF file I started from.</font><i>(See attached
file: otra.in)</i><i>(See attached file: otra.out)</i><br>
<font face="sans-serif" size="2">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</font><br>
<font face="sans-serif" size="2">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)</font><br>
<br>
<font face="sans-serif" size="2">will you please help with this?
probably some mistake from my side...</font><br>
<br>
<font face="sans-serif" size="2">thanks</font><br>
<br>
<font face="sans-serif" size="2">Manuel</font><br>
<font face="sans-serif" size="2">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</font><br>
<font face="sans-serif" size="2">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)</font><br>
<br>
<font face="sans-serif" size="2">Dr Manuel Pérez Jigato, Chargé
de Recherche<br>
Luxembourg Institute of Science and Technology (LIST)<br>
Materials Research and Technology (MRT)<br>
41 rue du Brill<br>
L-4422 BELVAUX<br>
Grand-Duché de Luxembourg<br>
Tel (+352) 47 02 61 - 584<br>
Fax (+352) 47 02 64<br>
e-mail <a class="moz-txt-link-abbreviated" href="mailto:manuel.perez@list.lu">manuel.perez@list.lu</a><br>
</font>
<br>
</p>
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</blockquote>
<br>
<br>
<div class="moz-signature">-- <br>
Michael J. Mehl<br>
Head, Center for Computational Materials Science<br>
Naval Research Laboratory Code 6390<br>
Washington DC
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