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 [email protected] >>> >>> ------------------------------------------------------------------------ >>> >>> >>> _______________________________________________ >>> Pw_forum mailing list >>> [email protected] >>> http://pwscf.org/mailman/listinfo/pw_forum >> >> _______________________________________________ >> Pw_forum mailing list >> [email protected] >> http://pwscf.org/mailman/listinfo/pw_forum >> > _______________________________________________ Pw_forum mailing list [email protected] http://pwscf.org/mailman/listinfo/pw_forum
