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I'm quite new to spin-polarized DFT and have been trying to study $BaVSe_{3}$ which is ferromagnetic (with a Final Magnetic Moment of 0.052 Bohr Mag) as per the Materials Project repository [picture 1]. The Vanadium carries the moment as per this paper. But when a spin-polarized SCF was executed [as per the input file 1], the total magnetization was obtained as 1.95 Bohr Mag/cell. The two values are quite different so am I doing something wrong here? Or are these values different things?

I'm using the primitive cell CIF file obtained from the Materials Project repo [Cif file] and have converted it directly into the Quantum ESPRESSO input file using cif2cell

Picture 1

enter image description here

input file 1

&CONTROL
  calculation = 'scf'
  outdir = './outdir'
  verbosity = 'low'
  tprnfor = .true.
  tstress = .true.
  pseudo_dir = '.'
/
&SYSTEM
  ibrav = 0
  A =    7.02663
  nat = 10
  ntyp = 3
  ecutwfc = 55
  ecutrho = 650
  nspin = 2
  starting_magnetization(1) = 1
  starting_magnetization(2) = 0
  starting_magnetization(3) = 0

  occupations = 'smearing'
  smearing = 'mv'
  degauss = 0.005d0      
/
&ELECTRONS
  conv_thr = 1e-8
  mixing_beta = 0.7d0
/
CELL_PARAMETERS {alat}
  0.866025403784439  -0.500000000000000   0.000000000000000 
  0.000000000000000   1.000000000000000   0.000000000000000 
  0.000000000000000   0.000000000000000   0.857028191323579 
ATOMIC_SPECIES
  V    50.94150  V.pbe-spnl-kjpaw_psl.1.0.0.UPF
  Ba   137.32700 Ba.pbe-spn-kjpaw_psl.1.0.0.UPF
  Se   78.96000  Se.pbe-dn-kjpaw_psl.1.0.0.UPF
ATOMIC_POSITIONS {crystal}
  Ba   0.333333333333333   0.666666666666667   0.250000000000000 
  Ba   0.666666666666667   0.333333333333333   0.750000000000000 
  Se   0.833430000000000   0.666666666666667   0.250000000000000 
  Se   0.833236666666667   0.166570000000000   0.250000000000000 
  Se   0.166570000000000   0.833236666666667   0.750000000000000 
  Se   0.333333333333333   0.166763333333333   0.250000000000000 
  Se   0.166763333333333   0.333333333333333   0.750000000000000 
  Se   0.666666666666667   0.833430000000000   0.750000000000000 
  V   0.000000000000000   0.000000000000000   0.000000000000000 
  V   0.000000000000000   0.000000000000000   0.500000000000000 
K_POINTS {automatic}
  17 17 17 0 0 0
      

Picture 2

enter image description here

CIF file

# CIF file created by FINDSYM, version 7.1

data_findsym-output
_audit_creation_method FINDSYM
_cod_database_code               None
_cell_length_a    7.0266300000
_cell_length_b    7.0266300000
_cell_length_c    6.0220200000
_cell_angle_alpha 90.0000000000
_cell_angle_beta  90.0000000000
_cell_angle_gamma 120.0000000000
_cell_volume      257.4939303923

_symmetry_space_group_name_H-M "P 63/m 2/m 2/c"
_symmetry_Int_Tables_number 194
_space_group.reference_setting '194:-P 6c 2c'
_space_group.transform_Pp_abc a,b,c;0,0,0

loop_
_space_group_symop_id
_space_group_symop_operation_xyz
1 x,y,z
2 x-y,x,z+1/2
3 -y,x-y,z
4 -x,-y,z+1/2
5 -x+y,-x,z
6 y,-x+y,z+1/2
7 x-y,-y,-z
8 x,x-y,-z+1/2
9 y,x,-z
10 -x+y,y,-z+1/2
11 -x,-x+y,-z
12 -y,-x,-z+1/2
13 -x,-y,-z
14 -x+y,-x,-z+1/2
15 y,-x+y,-z
16 x,y,-z+1/2
17 x-y,x,-z
18 -y,x-y,-z+1/2
19 -x+y,y,z
20 -x,-x+y,z+1/2
21 -y,-x,z
22 x-y,-y,z+1/2
23 x,x-y,z
24 y,x,z+1/2

loop_
_atom_site_label
_atom_site_type_symbol
_atom_site_symmetry_multiplicity
_atom_site_Wyckoff_label
_atom_site_fract_x
_atom_site_fract_y
_atom_site_fract_z
_atom_site_occupancy
_atom_site_fract_symmform
Ba1 Ba   2 c 0.33333 0.66667 0.25000 1.00000 0,0,0    
V1  V    2 a 0.00000 0.00000 0.00000 1.00000 0,0,0    
Se1 Se   6 h 0.83343 0.66686 0.25000 1.00000 Dx,2Dx,0 

# end of cif
$\endgroup$
2
  • $\begingroup$ Your calculation is 1.95 Bohr Mag/cell. Is the Material Project data for the same system (cell?)? $\endgroup$
    – Camps
    Dec 4 '20 at 17:38
  • $\begingroup$ @Camps yes! This can be verified by looking at the CIF file and the input file. There are also instances in the paper that the material has a paramagnetic moment[ "The effective magnetic moment obtained from magnetic susceptibility in the paramagnetic regime is ueff = 1.40 uB" in the abstract] . Hence it begs the question if final moment in materials project is the total moment in the QE output file $\endgroup$ Dec 5 '20 at 1:29
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With the Materials Project and magnetism, it's often useful to download the structure directly and examine the magnetic moments on that structure, for example using the pymatgen Python code and the MPRester class, like so:

from pymatgen import MPRester
with MPRester() as mpr:  # you may need to supply your API key in the parentheses here, see materialsproject.org/dashboard
    struct = mpr.get_structure_by_material_id('mp-27363')
print(struct)

For this mp-id, this gives:

Full Formula (Ba2 V2 Se6)
Reduced Formula: BaVSe3
abc   :   7.035690   7.035690   5.902194
angles:  90.000000  90.000000 119.366265
Sites (10)
  #  SP           a         b         c    magmom
---  ----  --------  --------  --------  --------
  0  Ba    0.663884  0.336116  0.754951     0.001
  1  Ba    0.336116  0.663884  0.254951     0
  2  V     0.974167  0.025833  0.013237     1.029
  3  V     0.025833  0.974167  0.513237     1.041
  4  Se    0.329967  0.163445  0.243081    -0.061
  5  Se    0.836555  0.670033  0.243081    -0.063
  6  Se    0.829283  0.170717  0.274618    -0.082
  7  Se    0.163445  0.329967  0.743081    -0.06
  8  Se    0.670033  0.836555  0.743081    -0.062
  9  Se    0.170717  0.829283  0.774618    -0.083

This is in units of Bohr magneton, so the moment on the V is 1.029 µB, and the total moment per unit cell is 1.66 µB (sum(struct.site_properties['magmom'])).

Why the discrepancy?

Two reasons,

  1. in a DFT calculation, the magnetic moment you obtain is very sensitive to both your initial ordering (ferromagnetic, antiferromagnetic, etc.) and also your use of Hubbard U. It is very common that different authors use different U values. The Materials Project uses a U of 3.25 eV on vanadium. This can also be affected by pseudo potentials, which will necessarily be different between simulation code like Quantum Espresso and VASP.

  2. the discrepancy on Materials Project could be the result of our slow-but-steady progress towards more explicitly calculating magnetic order of materials (e.g. see this paper). While the values reported by MP from an individual calculation always remain the same, the values reported on the "material details page" can change from release to release as new calculations come in, and bug in this case is also possible (we strive for quality but it's a large database and bugs do happen, we encourage people to check our database release logs where we disclose issues as we discover them).

We (meaning, people working on the Materials Project) try to be responsive on matsci.org/materials-project if you want to ask a question directly, or you can contact us over email.

In conclusion, your value of 1.95 µB per cell actually seems fairly reasonable!

Hope this helps!

$\endgroup$
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