How to do a lattice parameter optimization

 

To install the vasp code click on the following link:

 How to install the Vasp code

 

To execute vasp code we need always 4 input files which are:

POSCAR     INCAR    KPOINTS  POTCAR

To get more information about the input files check the following link:

https://icme.hpc.msstate.edu/mediawiki/images/d/d2/LS14_VASP.pdf 

 

We will take the example of Silicon in the fcc structure

POSCAR

fcc Si:
 3.9
 0.5 0.5 0.0
 0.0 0.5 0.5
 0.5 0.0 0.5
   1
cartesian
0 0 0

     Fcc Si lattice constant of 3.9  .
    1 atom per unit cell.

 

INCAR

System = fcc Si
ISTART = 0 ; ICHARG = 2
ENCUT = 240
ISMEAR = 0; SIGMA = 0.1

    Initial charge density form overlapping atoms.
    Energy cutoff of 240 eV from POTCAR file.
 

KPOINTS

k-points
 0
Monkhorst Pack
 11 11 11
 0  0  0

 

POTCAR

Download the pseudopotential file from this link POTCAR

 

NB: you can download the input files from fccSi.tgz 

 

Calculation

First we need to do a scf calculation

/Si_Vopt> vasp_std

 

To do optimization we need to use the following script  loop.sh

 

#! /bin/bash
BIN=/path/to/your/vasp/executable
rm WAVECAR SUMMARY.fcc
for i in  3.5 3.6 3.7 3.8 3.9 4.0 4.1 4.2 4.3 ; do
cat >POSCAR <<!
fcc:
   $i
 0.5 0.5 0.0
 0.0 0.5 0.5
 0.5 0.0 0.5
   1
cartesian
0 0 0
!
echo "a= $i" ; $BIN
E=`awk '/F=/ {print $0}' OSZICAR` ; echo $i $E  >>SUMMARY.fcc
done
cat SUMMARY.fcc

 

Try to modify the path of vasp exectables

In my case BIN=/home/algerien1970/abinitio/vasp5.4.1/bin/vasp_std

/Si_Vopt> chmod +x loop.sh

./loop.sh

The output for the SUMMARY.fcc file within this example should look like this:

3.5 1 F= -.44256712E+01 E0= -.44233993E+01 d E =-.454388E-02
3.6 1 F= -.46614699E+01 E0= -.46600410E+01 d E =-.285796E-02
3.7 1 F= -.47979864E+01 E0= -.47959298E+01 d E =-.411323E-02
3.8 1 F= -.48645042E+01 E0= -.48630063E+01 d E =-.299564E-02
3.9 1 F= -.48773847E+01 E0= -.48758538E+01 d E =-.306176E-02
4.0 1 F= -.48487436E+01 E0= -.48481092E+01 d E =-.126878E-02
4.1 1 F= -.47852634E+01 E0= -.47844854E+01 d E =-.155599E-02
4.2 1 F= -.46936947E+01 E0= -.46922530E+01 d E =-.288339E-02
4.3 1 F= -.45831167E+01 E0= -.45811837E+01 d E =-.386598E-02

• To make a quick plot of SUMMARY.fcc try:

gnuplot
gnuplot> plot "SUMMARY.fcc" using ($1):($4) w lp

• The equilibrium lattice constant is found at roughly 3.9 Å.

• A quick look at the results:

Reference: https://www.vasp.at/wiki/index.php/Fcc_Si