In this tutorial, we will take the example of Aluminum Al. We need the cell file and a script to create the param file with different values of cutoff energy.

Al.cell

 %BLOCK LATTICE_ABC
2.86 2.86 2.86
60 60 60
%ENDBLOCK LATTICE_ABC

%BLOCK POSITIONS_ABS
Al     0   0   0
%ENDBLOCK POSITIONS_ABS

%block species_pot
   NCP19
%endblock species_pot

kpoint_mp_grid 12 12 12

symmetry_generate


cutoff-conv.sh

#!/bin/sh

NAME=Al

rm -f ${NAME}_CUTOFF.dat

for CUTOFF in  400 600 800 1000 1200 1400 1600 1800 2000

do

cat > ${NAME}.param << EOF
task     SinglePoint          ! The TASK keyword instructs CASTEP what to do
xc_functional   LDA_PW        ! Which exchange-correlation functional to use.
cut_off_energy  $CUTOFF       !
opt_strategy    speed         ! Choose algorithms for best speed
fine_grid_scale = 2
EOF


echo ${NAME}_${CUTOFF}
mpirun -np 4 castep.mpi ${NAME}

done

#analyse the runs and extract the values we need into a single results file
grep 'cut-off energy for mixing ' ${NAME}.castep | awk '{print $6}' > C
grep 'Final energy' ${NAME}.castep | awk '{print $5}' > E
paste C E >> ${NAME}_CUTOFF.dat

rm C E
echo '  '
echo 'finished with results in '${NAME}_CUTOFF.dat

Execution 

$ sh cutoff-conv.sh

Al_400
Al_600
Al_800
Al_1000
Al_1200
Al_1400
Al_1600
Al_1800
Al_2000
 
finished with results in Al_CUTOFF.dat

 

The content of the file Al_CUTOFF.dat is :

400.0    -192.7083844767
600.0    -192.7105862983
800.0    -192.7123204974
1000.    -192.7132612141
1200.    -192.7142051707
1400.    -192.7144481942
1600.    -192.7149716189
1800.    -192.7151728479
2000.    -192.7157828622

 

Plotting

To plot the results we need to use the following python script:

cutoff-conv.py

import matplotlib.pyplot as plt
from matplotlib import rcParamsDefault
import numpy as np

plt.rcParams["figure.dpi"]=150
plt.rcParams["figure.facecolor"]="white"

x, y = np.loadtxt('Al_CUTOFF.dat', delimiter='	', unpack=True)
plt.plot(x, y, "o-", markersize=5, label='Etot vs cut_off_energy')
plt.xlabel('cut_off_energy (eV)')
plt.ylabel('Etot (eV)')
plt.legend(frameon=False)
plt.show()


Before running the script we need to install matplotlib tool for plotting

$ sudo apt-get install python3-matplotlib  
$ python cutoff-conv.py

 

We get the following picture