fix srd command


In input script of srd directory in examples directory in LAMMPS package, fix srd command has been written in following form:

fix 1 all srd 1 NULL 1.0 0.25 49894 collision slip

Instead of groupbig_ID has been written NULL! Should groupbig have been defined?
For stochastic rotation dynamic we need to define two group:
One for srd particles (solvent particles) and another one for macromolecules that interact with srd particles.
In fix command above group_ID( ID for group of srd particles) is “all” but the only group has been defined with group command in input script is “empty”(group_ID)
I am confused. If it is possible, please help me. I think this program is not run in order to this problem.

There are 2 scripts in examples/srd.

in.srd.pure which is for SRD particle with no macromolecules, hence
the NULL for the big group
in.srd.misture which does have big particles and speficies a big group

The doc page for fix srd explains that you can do either.