Dear all

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:

1- Using cg instead of david algorithm.
2- Different starting magnetization
3- Different initial experimentally determined atom positions
4- Different pseudopotentials
5- Applying smearing
6- Gamma-centered vs. shifted k-points
7-Using ibrav = 4  and listing a and c, vs. using ibrav = 0 and providing
cell_parameters list

but nothing really seems to work.
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.

Best Regards,
Mostafa Youssef
AUC -  Egypt

# Input file for the primitive cell
&CONTROL
   calculation =   'vc-relax'   ,
   verbosity = 'high' ,
   restart_mode = 'from_scratch' ,
   prefix='fe2o3' ,
   outdir='/data/mostafa/Fe2O3/fix_magnet_strucutre2/scratch/' ,
   pseudo_dir = '/home/mostafa/data/Fe2O3/PP' ,
   tstress = .true.  ,
   tprnfor= .true.  ,
   nstep = 2000 ,
   etot_conv_thr = 7.7D-6  ,
   forc_conv_thr = 4.0D-5 ,
 /
 &SYSTEM
   ibrav = 0 ,
    nat  =  10 ,
   ntyp  =  3 ,
   nbnd  = 96 ,
  ecutwfc  =  90 ,
  ecutrho  =  1080 ,
  nosym =.true.
  occupations = 'fixed',
  nspin = 2 ,
  starting_magnetization(1)=1.0,
  starting_magnetization(2)=-1.0,
  tot_magnetization = 0
 /
 &ELECTRONS
                    diagonalization='david',
                    mixing_mode = 'plain' ,
                    mixing_beta = 0.5,
                    startingwfc = 'random',
                    conv_thr  =  1.0d-9  ,
  /
&ions
    ion_dynamics='bfgs'
/
&CELL
   cell_dynamics = 'bfgs' ,
   cell_dofree = 'all',
   press_conv_thr = 0.5 , ! This is the default
 /
CELL_PARAMETERS  angstrom
     5.4200000000000000    0.0000000000000000    0.0000000000000000
     3.0870500000000000    4.4549430000000000    0.0000000000000000
     3.0870500000000000    1.6166160000000000    4.1512740000000000
ATOMIC_SPECIES
Fe1  55.845 Fe.pbe-spn-rrkjus_psl.0.2.1.UPF
Fe2  55.845 Fe.pbe-spn-rrkjus_psl.0.2.1.UPF
O    15.999 O.pbe-n-rrkjus_psl.0.1.UPF
ATOMIC_POSITIONS crystal
Fe1   0.14500  0.14500  0.14500
Fe1   0.85500  0.85500  0.85500
Fe2   0.35500  0.35500  0.35500
Fe2   0.64500  0.64500  0.64500
O     0.75000  0.45800  0.04200
O     0.95800  0.25000  0.54200
O     0.45800  0.04200  0.75000
O     0.25000  0.54200  0.95800
O     0.04200  0.75000  0.45800
O     0.54200  0.95800  0.25000
K_POINTS automatic
  4 4 4  1 1 1
############################################################################
# Conventional cell
############################################################################
&CONTROL
   calculation =   'vc-relax'   ,
   verbosity = 'high' ,
   restart_mode = 'from_scratch' ,
   prefix='fe2o3' ,
   outdir='/data/mostafa/Fe2O3/fix_magnet_conventional_str3_gamma/scratch/'
,
   pseudo_dir = '/home/mostafa/data/Fe2O3/PP' ,
   tstress = .true.  ,
   tprnfor= .true.  ,
   nstep = 2000 ,
   etot_conv_thr = 7.7D-6  ,
   forc_conv_thr = 4.0D-5 ,
 /
 &SYSTEM
   ibrav = 0 ,
    nat  =  30 ,
   ntyp  =  3 ,
   nbnd  = 176 ,
  ecutwfc  =  90 ,
  ecutrho  =  1080 ,
  !!!nosym =.true.
  occupations = 'fixed',
  nspin = 2 ,
  starting_magnetization(1)= 1.0,
  starting_magnetization(2)= -1.0,
  !!starting_magnetization(3)= 0.0,
  tot_magnetization = 0
 /
 &ELECTRONS
                    diagonalization='david',
                    mixing_mode = 'plain' ,
                    mixing_beta = 0.5,
                    startingwfc = 'random',
                    conv_thr  =  1.0d-9  ,
  /
&ions
    ion_dynamics='bfgs'
/
&CELL
   cell_dynamics = 'bfgs' ,
   cell_dofree = 'all',
   press_conv_thr = 0.5 , ! This is the default
 /
CELL_PARAMETERS  angstrom
        5.0288000107         0.0000000000         0.0000000000
       -2.5144000053         4.3550685598         0.0000000000
        0.0000000000         0.0000000000        13.7302999496
ATOMIC_SPECIES
Fe1  55.845 Fe.pbe-spn-rrkjus_psl.0.2.1.UPF
Fe2  55.845 Fe.pbe-spn-rrkjus_psl.0.2.1.UPF
O    15.999 O.pbe-n-rrkjus_psl.0.1.UPF
ATOMIC_POSITIONS crystal
Fe1     0.000000000         0.000000000         0.144999996
Fe1     0.000000000         0.000000000         0.855000019
Fe2     0.000000000         0.000000000         0.355000019
Fe2     0.000000000         0.000000000         0.644999981
Fe2     0.666666687         0.333333343         0.478333324
Fe1     0.666666687         0.333333343         0.188333333
Fe2     0.666666687         0.333333343         0.688333333
Fe1     0.666666687         0.333333343         0.978333354
Fe1    0.333333343         0.666666687         0.811666667
Fe2     0.333333343         0.666666687         0.521666646
Fe1     0.333333343         0.666666687         0.021666670
Fe2     0.333333343         0.666666687         0.311666667
O     0.291999996         0.000000000         0.250000000
O     0.708000004         0.000000000         0.750000000
O     0.000000000         0.291999996         0.250000000
O     0.000000000         0.708000004         0.750000000
O     0.708000004         0.708000004         0.250000000
O     0.291999996         0.291999996         0.750000000
O     0.958666682         0.333333343         0.583333313
O     0.374666661         0.333333343         0.083333336
O     0.666666687         0.625333309         0.583333313
O     0.666666687         0.041333336         0.083333336
O     0.374666661         0.041333336         0.583333313
O     0.958666682         0.625333309         0.083333336
O     0.625333309         0.666666687         0.916666687
O     0.041333336         0.666666687         0.416666657
O     0.333333343         0.958666682         0.916666687
O     0.333333343         0.374666661         0.416666657
O     0.041333336         0.374666661         0.916666687
O     0.625333309         0.958666682         0.416666657

K_POINTS automatic
  4 4 2  0 0 0

###################################
Final magnetization from vc-relax of the conventional cell

 atom:    1    charge:   12.8867    magn:    3.3569    constr:    0.0000
     atom:    2    charge:   12.8867    magn:    3.3569    constr:    0.0000
     atom:    3    charge:   12.8867    magn:   -3.3569    constr:    0.0000
     atom:    4    charge:   12.8867    magn:   -3.3569    constr:    0.0000
     atom:    5    charge:   12.9309    magn:   -1.3993    constr:    0.0000
     atom:    6    charge:   12.9053    magn:    3.2614    constr:    0.0000
     atom:    7    charge:   12.9053    magn:   -3.2614    constr:    0.0000
     atom:    8    charge:   12.9309    magn:    1.3993    constr:    0.0000
     atom:    9    charge:   12.9053    magn:    3.2614    constr:    0.0000
     atom:   10    charge:   12.9309    magn:   -1.3993    constr:    0.0000
     atom:   11    charge:   12.9309    magn:    1.3993    constr:    0.0000
     atom:   12    charge:   12.9053    magn:   -3.2614    constr:    0.0000
     atom:   13    charge:    5.2738    magn:    0.0000    constr:    0.0000
     atom:   14    charge:    5.2738    magn:    0.0000    constr:    0.0000
     atom:   15    charge:    5.2738    magn:    0.0000    constr:    0.0000
     atom:   16    charge:    5.2738    magn:    0.0000    constr:    0.0000
     atom:   17    charge:    5.2738    magn:    0.0000    constr:    0.0000
     atom:   18    charge:    5.2738    magn:    0.0000    constr:    0.0000
     atom:   19    charge:    5.2642    magn:   -0.1646    constr:    0.0000
     atom:   20    charge:    5.2642    magn:    0.1646    constr:    0.0000
     atom:   21    charge:    5.2642    magn:   -0.1646    constr:    0.0000
     atom:   22    charge:    5.2642    magn:    0.1646    constr:    0.0000
     atom:   23    charge:    5.2642    magn:   -0.1646    constr:    0.0000
     atom:   24    charge:    5.2642    magn:    0.1646    constr:    0.0000
     atom:   25    charge:    5.2642    magn:    0.1646    constr:    0.0000
     atom:   26    charge:    5.2642    magn:   -0.1646    constr:    0.0000
     atom:   27    charge:    5.2642    magn:    0.1646    constr:    0.0000
     atom:   28    charge:    5.2642    magn:   -0.1646    constr:    0.0000
     atom:   29    charge:    5.2642    magn:    0.1646    constr:    0.0000
     atom:   30    charge:    5.2642    magn:   -0.1646    constr:
0.0000
_______________________________________________
users mailing list
users@lists.quantum-espresso.org
https://lists.quantum-espresso.org/mailman/listinfo/users

Reply via email to