In this tutorial, we do a volume optimization for the cubic structure as in the case of Silicon. We need to use 2 scripts: a shell script to do automatic calculations for different values of lattice parameter and a python script to do a fitting to the Birch-Murnagham equation.

The input files are Si.cell and Si.param. In the Si.cell file we need to use the lattice parameters as follows:

%BLOCK LATTICE_ABC
[units]
a b c
alpha beta gamma
%ENDBLOCK LATTICE_ABC

Si.cell

%block lattice_abc
3.78 3.78 3.78
60.0 60.0 60.0
%endblock lattice_abc

%block positions_frac
Si 0.00 0.00 0.00
Si 0.25 0.25 0.25
%endblock positions_frac

kpoint_mp_grid 4 4 4

symmetry_generate

 Si.param

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

 

 You may download the input files using command line:

$ wget https://raw.githubusercontent.com/jazairdz/Castep/refs/heads/main/Si.cell

$ wget https://raw.githubusercontent.com/jazairdz/Castep/refs/heads/main/Si.param

 

Scripts generate_EV.sh  and fit_Burch.py

You may download the 2 scripts using the command line

$ wget https://raw.githubusercontent.com/jazairdz/Castep/refs/heads/main/generate_EV.sh

$ wget https://raw.githubusercontent.com/jazairdz/Castep/refs/heads/main/fit_Burch.py

 

Execution 

$ sudo apt install dos2unix
$ dos2unix generate_EV.sh
$ bash generate_EV.sh
Generating energy-volume data for Si
   doing a= 3.60
   doing a= 3.65
   doing a= 3.70
   doing a= 3.75
   doing a= 3.80
   doing a= 3.85
   doing a= 3.90
   doing a= 3.95
   doing a= 4.00
finished with results in Si_EV.dat
NB each run contains 2 atoms

 This will create the file Si_EV.dat

 

$ python fit_Burch.py 
Enter name of (vol,energy) file to load : Si_EV.dat
Birch-Murnaghan fit parameters:
E0 =  -337.9959 eV
B0 =     0.5785 eV.ang^-3
B' =     4.1611 
V0 =    39.5975 ang^3

Bulk modulus =    92.6759 GPa

Graph of data and fit saved as BM_curve.png - please check all is OK
Plot of fit saved as BM_curve.png