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<p><font size="2" face="sans-serif">hello</font><br>
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<font size="2" face="sans-serif">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 size="2" face="sans-serif">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 size="2" face="sans-serif">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>
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<font size="2" face="sans-serif">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 size="2" face="sans-serif">whilst all the oxygen atoms have their z coordinate not fixed by symmetry</font><br>
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<font size="2" face="sans-serif">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>
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<font size="2" face="sans-serif"> atom 1 type 1 force = 0.00000000 0.00000000 0.00000000</font><br>
<font size="2" face="sans-serif"> atom 2 type 1 force = 0.00000000 0.00000000 -0.00022475</font><br>
<font size="2" face="sans-serif"> atom 3 type 1 force = -0.00000000 -0.00000000 0.00022475</font><br>
<font size="2" face="sans-serif"> atom 4 type 2 force = -0.00000000 0.00000000 0.00000000</font><br>
<font size="2" face="sans-serif"> atom 5 type 2 force = 0.00000000 0.00000000 0.00035717</font><br>
<font size="2" face="sans-serif"> atom 6 type 2 force = 0.00000000 -0.00000000 -0.00035717</font><br>
<font size="2" face="sans-serif"> atom 7 type 3 force = 0.00000000 0.00000000 0.00241988</font><br>
<font size="2" face="sans-serif"> atom 8 type 3 force = 0.00000000 0.00000000 -0.00241988</font><br>
<font size="2" face="sans-serif"> atom 9 type 3 force = -0.00000000 0.00000000 0.00239382</font><br>
<font size="2" face="sans-serif"> atom 10 type 3 force = 0.00000000 0.00000000 0.00239626</font><br>
<font size="2" face="sans-serif"> atom 11 type 3 force = 0.00000000 0.00000000 -0.00239626</font><br>
<font size="2" face="sans-serif"> atom 12 type 3 force = 0.00000000 -0.00000000 -0.00239382</font><br>
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<font size="2" face="sans-serif">the first 3 lines correspond to Cu, the next 3 lines are Cr, and the last 6 lines are oxygen</font><br>
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<font size="2" face="sans-serif">According to the site symmetries, only the z components of the force on the oxygen atoms (bottom six lines)</font><br>
<font size="2" face="sans-serif">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 size="2" face="sans-serif">as far as I can see, they should be zero</font><br>
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<font size="2" face="sans-serif">This problem appears when I run scf under ibrav 0 and also when I use ibrav 4.</font><br>
<font size="2" face="sans-serif">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 size="2" face="sans-serif">by means of cif2cell, which gives the ibrav 0 option. Still, the problem of non-zero forces persists.</font><br>
<font size="2" face="sans-serif">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 size="2" face="sans-serif">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 size="2" face="sans-serif">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>
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<font size="2" face="sans-serif">will you please help with this? probably some mistake from my side...</font><br>
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<font size="2" face="sans-serif">thanks</font><br>
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<font size="2" face="sans-serif">Manuel</font><br>
<font size="2" face="sans-serif">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 size="2" face="sans-serif">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>
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<font size="2" face="sans-serif">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 manuel.perez@list.lu<br>
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