Dear Aniruddh Vashisth ,

For example we have three groups in our LAMMPS simulation:

group firstpolymer type 1

group secondpolymer type 2

group pore type 3

then you can fix one group by using this command (I'm gonna fix second group):

fix 2 secondpolymer setforce 0 0 0

then when you're satisfied with equilibrium or anything else you can unfix it like this:

unfix 2

Please be careful, the number in front of unfix is the number of corresponding fix command not the group type.

Obviously, you can use setforce command for pulling the other end. Please check the velocity command too.

I hope you find it useful.

Good luck.

Thank you Mohammadreza!