[lammps-users] difference between comm and reverse_com

atom->tag[i] is the global ID of an atom. I is a local
index, for where the atom is stored on that proc (node?)

Forward and reverse comm only communicated info between
owned and ghost atoms. Not for all atoms. If you want
to get atom IDs for all atoms in the system (ill-advised, not
parallel, could require huge memory), then you'd have to
write MPI code to do it yourself - e.g. MPI_Allgather()