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<div><div><div><div><div><div>Dear all<br><br></div><div>Hematite
(alpha-Fe2O3) crystallizes in the corundum structure. The primitive
cell is rhombohedral containing 2 formula units, whereas the
conventional cell is hexagonal containing 6 formula units. I'm trying to
model the antiferromagnetic phase of hematite with spin ordering (+--+)
in the primitive cell. Relaxation (or variable cell relaxation) works
nicely for the primitive cell and I obtain a reasonable final structure
and magnetic moments on iron ions and zero magnetic moment in oxygen
ions. This is consistent with literature. Note that all iron ions are
equivalent crystallographically and all oxygen ions are
crystallographyically. However, when I try to relax (or vc-relax) the
corresponding conventional cell, some symmetry breaking occurs and few
iron ions develop magnetic moment different than the rest. Additionally
some oxygen ions develop magnetic moment as well. The final result is
antiferromagnetic still, but of course the structure is not consistent
with prior reports and is not consistent with primitive cell
calculations. Also the energy/Fe2O3 for the conventional cell is higher
than the primitive cell. I tried many things to avoid this
strange-looking local minimum such as:<br><br></div><div>1- Using cg instead of david algorithm.<br></div><div>2- Different starting magnetization<br></div><div>3- Different initial experimentally determined atom positions<br></div><div>4- Different pseudopotentials<br></div><div>5- Applying smearing <br></div><div>6- Gamma-centered vs. shifted k-points<br></div><div>7-Using ibrav = 4 and listing a and c, vs. using ibrav = 0 and providing cell_parameters list<br><br></div><div>but nothing really seems to work.<br></div><div>Any
suggestions are thoughts arevery appreciated. I provide below the input
files for both primitive and conventional cells and I'm using version
6.2.1. I also provide the final magnetization of the ions in the
conventional cell.<br><br></div><div>Best Regards,<br></div></div>Mostafa Youssef<br></div>AUC - Egypt<br><br></div># Input file for the primitive cell<br>&CONTROL<br> calculation = 'vc-relax' ,<br> verbosity = 'high' ,<br> restart_mode = 'from_scratch' ,<br> prefix='fe2o3' ,<br> outdir='/data/mostafa/Fe2O3/<wbr>fix_magnet_strucutre2/scratch/<wbr>' ,<br> pseudo_dir = '/home/mostafa/data/Fe2O3/PP' ,<br> tstress = .true. ,<br> tprnfor= .true. ,<br> nstep = 2000 ,<br> etot_conv_thr = 7.7D-6 ,<br> forc_conv_thr = 4.0D-5 ,<br> /<br> &SYSTEM<br> ibrav = 0 ,<br> nat = 10 , <br> ntyp = 3 , <br> nbnd = 96 ,<br> ecutwfc = 90 ,<br> ecutrho = 1080 , <br> nosym =.true.<br> occupations = 'fixed',<br> nspin = 2 ,<br> starting_magnetization(1)=1.0,<br> starting_magnetization(2)=-1.<wbr>0,<br> tot_magnetization = 0<br> /<br> &ELECTRONS<br> diagonalization='david',<br> mixing_mode = 'plain' ,<br> mixing_beta = 0.5,<br> startingwfc = 'random',<br> conv_thr = 1.0d-9 , <br> /<br>&ions<br> ion_dynamics='bfgs'<br>/<br>&CELL<br> cell_dynamics = 'bfgs' ,<br> cell_dofree = 'all',<br> press_conv_thr = 0.5 , ! This is the default<br> /<br>CELL_PARAMETERS angstrom<br> 5.4200000000000000 0.0000000000000000 0.0000000000000000<br> 3.0870500000000000 4.4549430000000000 0.0000000000000000<br> 3.0870500000000000 1.6166160000000000 4.1512740000000000 <br>ATOMIC_SPECIES<br>Fe1 55.845 Fe.pbe-spn-rrkjus_psl.0.2.1.<wbr>UPF<br>Fe2 55.845 Fe.pbe-spn-rrkjus_psl.0.2.1.<wbr>UPF<br>O 15.999 O.pbe-n-rrkjus_psl.0.1.UPF<br>ATOMIC_POSITIONS crystal <br>Fe1 0.14500 0.14500 0.14500<br>Fe1 0.85500 0.85500 0.85500<br>Fe2 0.35500 0.35500 0.35500<br>Fe2 0.64500 0.64500 0.64500<br>O 0.75000 0.45800 0.04200<br>O 0.95800 0.25000 0.54200<br>O 0.45800 0.04200 0.75000<br>O 0.25000 0.54200 0.95800<br>O 0.04200 0.75000 0.45800<br>O 0.54200 0.95800 0.25000 <br>K_POINTS automatic <br> 4 4 4 1 1 1<br>##############################<wbr>##############################<wbr>################<br></div># Conventional cell<br>##############################<wbr>##############################<wbr>################<br>&CONTROL<br> calculation = 'vc-relax' ,<br> verbosity = 'high' ,<br> restart_mode = 'from_scratch' ,<br> prefix='fe2o3' ,<br> outdir='/data/mostafa/Fe2O3/<wbr>fix_magnet_conventional_str3_<wbr>gamma/scratch/' ,<br> pseudo_dir = '/home/mostafa/data/Fe2O3/PP' ,<br> tstress = .true. ,<br> tprnfor= .true. ,<br> nstep = 2000 ,<br> etot_conv_thr = 7.7D-6 ,<br> forc_conv_thr = 4.0D-5 ,<br> /<br> &SYSTEM<br> ibrav = 0 ,<br> nat = 30 , <br> ntyp = 3 , <br> nbnd = 176 ,<br> ecutwfc = 90 ,<br> ecutrho = 1080 , <br> !!!nosym =.true.<br> occupations = 'fixed',<br> nspin = 2 ,<br> starting_magnetization(1)= 1.0,<br> starting_magnetization(2)= -1.0,<br> !!starting_magnetization(3)= 0.0,<br> tot_magnetization = 0<br> /<br> &ELECTRONS<br> diagonalization='david',<br> mixing_mode = 'plain' ,<br> mixing_beta = 0.5,<br> startingwfc = 'random',<br> conv_thr = 1.0d-9 , <br> /<br>&ions<br> ion_dynamics='bfgs'<br>/<br>&CELL<br> cell_dynamics = 'bfgs' ,<br> cell_dofree = 'all',<br> press_conv_thr = 0.5 , ! This is the default<br> /<br>CELL_PARAMETERS angstrom<br> 5.0288000107 0.0000000000 0.0000000000<br> -2.5144000053 4.3550685598 0.0000000000<br> 0.0000000000 0.0000000000 13.7302999496<br>ATOMIC_SPECIES<br>Fe1 55.845 Fe.pbe-spn-rrkjus_psl.0.2.1.<wbr>UPF<br>Fe2 55.845 Fe.pbe-spn-rrkjus_psl.0.2.1.<wbr>UPF<br>O 15.999 O.pbe-n-rrkjus_psl.0.1.UPF<br>ATOMIC_POSITIONS crystal <br>Fe1 0.000000000 0.000000000 0.144999996<br>Fe1 0.000000000 0.000000000 0.855000019<br>Fe2 0.000000000 0.000000000 0.355000019<br>Fe2 0.000000000 0.000000000 0.644999981<br>Fe2 0.666666687 0.333333343 0.478333324<br>Fe1 0.666666687 0.333333343 0.188333333<br>Fe2 0.666666687 0.333333343 0.688333333<br>Fe1 0.666666687 0.333333343 0.978333354<br>Fe1 0.333333343 0.666666687 0.811666667<br>Fe2 0.333333343 0.666666687 0.521666646<br>Fe1 0.333333343 0.666666687 0.021666670<br>Fe2 0.333333343 0.666666687 0.311666667<br>O 0.291999996 0.000000000 0.250000000<br>O 0.708000004 0.000000000 0.750000000<br>O 0.000000000 0.291999996 0.250000000<br>O 0.000000000 0.708000004 0.750000000<br>O 0.708000004 0.708000004 0.250000000<br>O 0.291999996 0.291999996 0.750000000<br>O 0.958666682 0.333333343 0.583333313<br>O 0.374666661 0.333333343 0.083333336<br>O 0.666666687 0.625333309 0.583333313<br>O 0.666666687 0.041333336 0.083333336<br>O 0.374666661 0.041333336 0.583333313<br>O 0.958666682 0.625333309 0.083333336<br>O 0.625333309 0.666666687 0.916666687<br>O 0.041333336 0.666666687 0.416666657<br>O 0.333333343 0.958666682 0.916666687<br>O 0.333333343 0.374666661 0.416666657<br>O 0.041333336 0.374666661 0.916666687<br>O 0.625333309 0.958666682 0.416666657<br><br>K_POINTS automatic <br> 4 4 2 0 0 0<br><br>##############################<wbr>#####<br></div>Final magnetization from vc-relax of the conventional cell<br><br> atom: 1 charge: 12.8867 magn: 3.3569 constr: 0.0000<br> atom: 2 charge: 12.8867 magn: 3.3569 constr: 0.0000<br> atom: 3 charge: 12.8867 magn: -3.3569 constr: 0.0000<br> atom: 4 charge: 12.8867 magn: -3.3569 constr: 0.0000<br> atom: 5 charge: 12.9309 magn: -1.3993 constr: 0.0000<br> atom: 6 charge: 12.9053 magn: 3.2614 constr: 0.0000<br> atom: 7 charge: 12.9053 magn: -3.2614 constr: 0.0000<br> atom: 8 charge: 12.9309 magn: 1.3993 constr: 0.0000<br> atom: 9 charge: 12.9053 magn: 3.2614 constr: 0.0000<br> atom: 10 charge: 12.9309 magn: -1.3993 constr: 0.0000<br> atom: 11 charge: 12.9309 magn: 1.3993 constr: 0.0000<br> atom: 12 charge: 12.9053 magn: -3.2614 constr: 0.0000<br> atom: 13 charge: 5.2738 magn: 0.0000 constr: 0.0000<br> atom: 14 charge: 5.2738 magn: 0.0000 constr: 0.0000<br> atom: 15 charge: 5.2738 magn: 0.0000 constr: 0.0000<br> atom: 16 charge: 5.2738 magn: 0.0000 constr: 0.0000<br> atom: 17 charge: 5.2738 magn: 0.0000 constr: 0.0000<br> atom: 18 charge: 5.2738 magn: 0.0000 constr: 0.0000<br> atom: 19 charge: 5.2642 magn: -0.1646 constr: 0.0000<br> atom: 20 charge: 5.2642 magn: 0.1646 constr: 0.0000<br> atom: 21 charge: 5.2642 magn: -0.1646 constr: 0.0000<br> atom: 22 charge: 5.2642 magn: 0.1646 constr: 0.0000<br> atom: 23 charge: 5.2642 magn: -0.1646 constr: 0.0000<br> atom: 24 charge: 5.2642 magn: 0.1646 constr: 0.0000<br> atom: 25 charge: 5.2642 magn: 0.1646 constr: 0.0000<br> atom: 26 charge: 5.2642 magn: -0.1646 constr: 0.0000<br> atom: 27 charge: 5.2642 magn: 0.1646 constr: 0.0000<br> atom: 28 charge: 5.2642 magn: -0.1646 constr: 0.0000<br> atom: 29 charge: 5.2642 magn: 0.1646 constr: 0.0000<br> atom: 30 charge: 5.2642 magn: -0.1646 constr: 0.0000
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