In this tutorial we will calculate the Band Structure of the antiferromagnetic Iron Oxide feO using PBE and PBE+U methods. We need 2 input files .cell and .param.


PBE Method

FeO-pbe.cell

! FeO.cell
%BLOCK LATTICE_CART
       1.768531594289456       0.000000000000001       5.002162732258922
      -0.884265797144728       1.531593288050063       5.002162732258921
      -0.884265797144728      -1.531593288050063       5.002162732258922
%ENDBLOCK LATTICE_CART                  

%BLOCK POSITIONS_FRAC
  O   0.75   0.750  -1.250
  O  -0.75  -0.750   1.250           
 Fe   0.00   0.000  -0.000 spin=-4.0
 Fe   1.50  -0.500  -0.500 spin=4.0
%ENDBLOCK POSITIONS_FRAC

kpoints_mp_grid: 6 6 6

symmetry_generate

%block spectral_kpoint_path
0.500 0.500 0.000
0.000 0.000 0.000
0.500 0.500 0.500
0.000 0.500 0.000
0.000 0.000 0.000
%endblock spectral_kpoint_path

 

FeO-pbe.param

task            spectral      ! The TASK keyword instructs CASTEP what to do
spectral_task   bandstructure !
xc_functional   PBE           ! Which exchange-correlation functional to use.
cut_off_energy  600 eV        !
opt_strategy    speed         ! Choose algorithms for best speed
nextra_bands : 17
spin_polarised  true          ! Run a spin polarised calculation

 

Execution

For serial calculation

/FeO-pbe-bands$ castep.serial FeO-pbe

For parallel calculation 

/FeO--pbe-bands$ mpirun -np 4 castep.mpi FeO-pbe

 Plotting 

/FeO-pbe-bands$ dispersion.pl -sym fcc -xg FeO-pbe

 A PBE calculation incorrectly finds FeO to be a metal.

 

PBE+U Method

FeO-pbe-u.cell

! FeO-u.cell
%BLOCK LATTICE_CART
       1.768531594289456       0.000000000000001       5.002162732258922
      -0.884265797144728       1.531593288050063       5.002162732258921
      -0.884265797144728      -1.531593288050063       5.002162732258922
%ENDBLOCK LATTICE_CART                  

%BLOCK POSITIONS_FRAC
  O   0.75   0.750  -1.250
  O  -0.75  -0.750   1.250           
 Fe   0.00   0.000  -0.000 spin=-4.0
 Fe   1.50  -0.500  -0.500 spin=4.0
%ENDBLOCK POSITIONS_FRAC

kpoints_mp_grid: 6 6 6

symmetry_generate

%block spectral_kpoint_path
0.500 0.500 0.000
0.000 0.000 0.000
0.500 0.500 0.500
0.000 0.500 0.000
0.000 0.000 0.000
%endblock spectral_kpoint_path

%block hubbard_u
Fe 1 d: 2.5
Fe 2 d: 2.5
%endblock hubbard_u

For the param file we need only to make copy with the new name as follows: 

cp FeO-pbe.param FeO-pbe-u.param

 

 

Execution

For serial calculation

/FeO-pbe-u-bands$ castep.serial FeO-pbe-u

For parallel calculation 

/FeO--pbe-u-bands$ mpirun -np 4 castep.mpi FeO-pbe-u

 Plotting 

/FeO-pbe-u-bands$ dispersion.pl -sym fcc -xg FeO-pbe-u

 


Note

Adding a Hubbard U term to the calculation opens the band gap and FeO is (correctly) predicted to be an antiferromangetic insulator.

 

Reference:  https://castep-docs.github.io/castep-docs/tutorials/Bands_and_DOS/magnetic/

 https://www.castep.org/features/capabilities/hamiltonian