Groups are not fixed. It didn't work because you only defined the group once. If you would like to update it, it can be done using a loop: http://lammps.sandia.gov/doc/jump.html .
16.06.2013, 11:02, "Saikat Banerjee" <[email protected]>:
Perhaps I phrased my answer poorly. What I meant is that once you start a particular run command, you can't change groups until it finishes.
It's true that you can break your run into smaller ones, but the groups will still be fixed for the duration of your smaller runs. If the atoms move significantly on the timescale of these smaller runs, then the group will likely not correspond to the actual atoms in the region, so you'll miss some (or count extra ones).
dump_modify region does not suffer from this problem.