Rmsd calculation at localized region

Respected Lammps Users, I am using the lammps version march 2020, During the sintering process of two nano particles I want to calculate the the slope of rmsd/time at the localized region where the sintering necks develop to check for localized interatomic diffusivity. Before the start of simulation procedure this region is empty as I put the space of 2 angstrom between the two particles supposed to be sintered. or is it fine to use command in the thermostyle? Which lammps command can perform this job?
Thanks for cooperation

The answer @Germain wrote 7 hours ago on your previous post perfectly applies here too:

3 Likes