How to fix a fraction of atoms in a Simulation box


I have a High entropy alloy system consisting of 4 elements in a 20 20 20 simulation box
1.I have started of with an initial completely FCC structure then i started melting the system
2.In the due course i would like to fix the position of 0.2% atoms at the center of the simulation box through out the silmulation durng melting process
3.Then i start solidifying it to room temperature
3.I would like to know which command i use for that ?

please don’t use the term “fix” in this context. it collides with the keyword “fix” in LAMMPS and in English it actually doesn’t exactly mean what you want to say.
“Immobilize” would be a more accurate term.

here is an approach that could work.
you double up on the number of atom types, but assign the same element labels and parameters to 2 atom types each.
then you can use the set command with the type/fraction option to change a desired fraction of atoms in the desired region to the second type for that same element. those would be the immobilized atoms.
now you create a group of atoms based only on the first set of types and use that group to assign velocities and do time integration and thermostatting.
since the other atoms are not time integrated, they will not move. if you switch time integration to group all, all atoms can move again.