In a previous tutorial, we have calculated the phonon properties of a cubic structure. Now, we will calculate the phonon properties of the hexagonal structure of the Boron Nitride BN.

We need the following input files:

 Input files

 BN_DFPT.cell 

%block LATTICE_ABC
   2.536000   2.536000     4.199000  ! a, b, c
  90.000000  90.000000   120.000000  ! alpha, beta, gamma
%endblock LATTICE_ABC

%block POSITIONS_FRAC
  B   2/3   1/3    0.000000  ! Atom co-ordinates in crystallographic
  N   2/3   1/3    0.374536  ! fractional system.
  B   1/3   2/3    0.500000
  N   1/3   2/3    0.874536
%endblock POSITIONS_FRAC

%block SPECIES_POT
B   B_00.recpot              ! File names of pseudopotential to use for B
N   N_00.recpot
%endblock SPECIES_POT

kpoints_mp_grid 5 5 6      ! Generate MP grid of electronic k-points.

phonon_kpoint_mp_grid 3 3 4     ̣! Generate MP grid of phonon q-points
phonon_kpoint_mp_offset : INCLUDE_GAMMA

symmetry_generate            ! Find symmetry operations of crystal structure

 

BN_DFPT.param 

task             : PHONON    ! Choose a phonon calculation
phonon_method    : DFPT      ! It's the default method
xc_functional    : LDA
opt_strategy     : SPEED     ! Optimise for speed over memory saving
cut_off_energy   : 700.0 eV  ! Plane-wave cutoff for this system
elec_method      : DM        ! Use fast DM solver even for insulating system
phonon_sum_rule  : TRUE      ! Enforce acoustic sum rule on calculated DM
calculate_born_charges : false
phonon_calc_lo_to_splitting : false
efield_calc_ion_permittivity : false

 

BN_Interpolate.cell

%block LATTICE_ABC
   2.536000   2.536000     4.199000  ! a, b, c
  90.000000  90.000000   120.000000  ! alpha, beta, gamma
%endblock LATTICE_ABC

%block POSITIONS_FRAC
  B   2/3   1/3    0.000000  ! Atom co-ordinates in crystallographic
  N   2/3   1/3    0.374536  ! fractional system.
  B   1/3   2/3    0.500000
  N   1/3   2/3    0.874536
%endblock POSITIONS_FRAC

%block SPECIES_POT
B   B_00.recpot              ! File names of pseudopotential to use for B
N   N_00.recpot
%endblock SPECIES_POT

kpoints_mp_grid 5 5 6      ! Generate MP grid of electronic k-points.

phonon_kpoint_mp_grid 3 3 4     ̣! Generate MP grid of phonon q-points
phonon_kpoint_mp_offset : INCLUDE_GAMMA

#phonon_fine_kpoint_mp_grid 15 15 15

#%block PHONON_FINE_KPOINT_PATH
#0.0 0.0 0.0
#0.5 0.0 0.0
#1/3 1/3 0.0
#0.0 0.0 0.0
#0.0 0.0 0.5
#0.5 0.0 0.5
#1/3 1/3 0.5
#0.0 0.0 0.5
#%endblock PHONON_FINE_KPOINT_PATH

symmetry_generate           ! Find symmetry operations of crystal structure

 BN_Interpolate.param

task : Phonon
phonon_method    : DFPT      ! It's the default method
fix_occupancy : true
continuation : BN_DFPT.check
phonon_fine_method : interpolate

 You can download the PP files from the following links:

$ wget https://raw.githubusercontent.com/jazairdz/Castep-PP/refs/heads/main/B_00.recpot

$ wget https://raw.githubusercontent.com/jazairdz/Castep-PP/refs/heads/main/N_00.recpot

 

Execution and Plotting  

$ mpirun -np 4 castep.mpi BN_DFPT

 

For Band Structure or Dispersion calculation, we need to make a copy of Dispersion cell file 

$ cp BN_Interpolate.cell BN_Dispersion.cell

and to uncheck the following lines of the BN_Dispersion.cell file 

%block PHONON_FINE_KPOINT_PATH
0.0 0.0 0.0
0.5 0.0 0.0
1/3 1/3 0.0
0.0 0.0 0.0
0.0 0.0 0.5
0.5 0.0 0.5
1/3 1/3 0.5
0.0 0.0 0.5
%endblock PHONON_FINE_KPOINT_PATH

and running the following commands:

$ mpirun -np 4 castep.mpi BN_Dispersion
$ dispersion.pl -sym hexagonal -xg BN_Dispersion.phonon 

 

For DOS calculation, we need to make a copy of DOS cell file 

$ cp BN_Interpolate.cell BN_DoS.cell

and to uncheck the following lines of the BN_DoS.cell file 

phonon_fine_kpoint_mp_grid 15 15 15

and running the following commands:

$ mpirun -np 4 castep.mpi BN_DoS
$ dos.pl -xg BN_DoS.phonon -w 0.3

 

We get the following plots