In this tutorial we will show you the cell and param files needed to plot bandstructure plots for various the semiconductor Silicon.

 

We need 2 files

si-bands.cell

 %block lattice_abc
3.8 3.8 3.8
60 60 60
%endblock lattice_abc
!
! Atomic co-ordinates for each species.
! These are in fractional co-ordinates wrt to the cell.
!
%block positions_frac
Si 0.00 0.00 0.00
Si 0.25 0.25 0.25
%endblock positions_frac
!
! Analyse structure to determine symmetry
!
symmetry_generate
!
! Specify M-P grid dimensions for electron wavevectors (K-points)
!
kpoint_mp_grid 4 4 4

! Specify a path through the Brillouin Zone to compute the band structure.
!
%block spectral_kpoint_path
0.5 0.25 0.75    ! W
0.5 0.5 0.5      ! L
0.0 0.0  0.0     ! Gamma
0.5 0.0 0.5      ! X
0.5 0.25 0.75    ! W
0.375 0.375 0.75 ! K
%endblock spectral_kpoint_path

si-bands.param

task            spectral      ! The TASK keyword instructs CASTEP what to do
spectral_task   bandstructure !
xc_functional   LDA           ! Which exchange-correlation functional to use.
cut_off_energy  500 eV        !
opt_strategy    speed         ! Choose algorithms for best speed

 

Note

There are 2 ways to use the pseudopotentials:

- Using an external pseudopotential with extention .usp

- Using an internal pseudopotential created by the code during the execution according to type mentioned  in the param file which we will do it in this tutorial.


Execution

For serial calculation

/Si-bands$ castep.serial si-bands

For parallel calculation 

/Si-bands$ mpirun -np 4 castep.mpi si-bands

 

We will get the following files

/Si-bands$ ls
si-bands  si-bands.bands  si-bands.castep  si-bands.cell   si-bands.cst_esp 
si-bands.param   si-bands.0001.spec  si-bands.bib    si-bands.castep_bin 
si-bands.check  si-bands-out.cell  Si_C19_LDA_OTF.usp 

 

Plotting 

/Si-bands$ dispersion.pl -sym fcc -xg si-bands.bands

 We will get the following picture


Note

Silicon is a semiconductor with a small gap between the occupied (blue) and unoccupied states (brown). The gap is indirect as the valence band maximum is as Gamma, but the conduction band minimum lies between Gamma and X.


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