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:
helloI 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 systemBy 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 symmetryThe 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, ieatom 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 -0.00239382the first 3 lines correspond to Cu, the next 3 lines are Cr, and the last 6 lines are oxygenAccording 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 zeroThis 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 ManuelPS 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 [email protected] _______________________________________________ Pw_forum mailing list [email protected] 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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