Dear Afza,

before defining lattice constant, define a variable like rn to read its value from a file like this:

variable rn file constant.txt

Then define your lattice:

lattice fcc ${rn}

For changing value of rn in the file you can write a simple bash script which every time it changes the lattice constant, runs your code saves your results then changes the constant again.

I attached the code below. Please change the code below, this is just a sample code:

#!/usr/bin/env bash

constant=5

counter=1

while [ $counter -le 4 ]

do

echo $constant > constant.txt #It writes constant value to a file and in in.lattice you forced lammps to read it

echo $(lammps -in in.lattice)

mkdir $counter/

mv log.lammps energy.dat $counter/ #Write your results here for moving to a folder after each run

((constant++))

((counter++)) > /dev/null

done

A while back, I loaded a number of simple LAMMPS scripts online. The first ones I did were to calculate energy as a function of lattice parameter. You might want to read through tutorials 1 and 2.

https://icme.hpc.msstate.edu/mediawiki/index.php/LAMMPS_tutorials

Thank you!