srd example

If you look at srd example, you see these command as follows:

pair_style lj/cut 2.5
pair_coeff 1 1 1.0 1.0 2.5
pair_coeff 2 2 0.0 1.0 0.0
pair_coeff 1 2 1.0 1.0 0.5
If interaction between small particles and big particles are computed by fix srd command so why these commands are defined in input file (in.srd.mixture)?
Best Regards
Samira

Two reasons.

At that point in the script, there are no SRD particles,
it is just equilibrating the big particles.

Second, the system has 2 types of particles,
The big particles are type 1. You have to define
LJ coeffs for all type/type interactions. The 2/2
and 1/2 interactions have epsilon = 0.0, so
there is effectively no LJ interactions with type 2
particles (the SRD particles) (NOTE: you
mistyped them below, in the script the 1/2 eps = 0.0).

I repeat, all SRD/big interactions are computed
by the fix srd command.

Steve

I copied those commands from srd input file as an example in lammps package and pasted. In that input file LJ interactions are defined between big/big and big/small particles with epsilon=!0.
If all SRD/big interactions are computed by fix srd command so what it is required to defined that commands?

If particles of wall are not defined as big particles and just polymer is defined as big particles, how i can compute interaction between srd/wall particles such that wall particles are not in big group and i think no longer interaction between srd/wall particles are computed by srd command so what should i do?
thanks
samira

Comments below.

Steve

I copied those commands from srd input file as an example in lammps package
and pasted. In that input file LJ interactions are defined between big/big
and big/small particles with epsilon=!0.

What file is that? Not examples/srd/in.srd.mixture. In that one
all epsilons involving type 2 particles (SRD particles) are 0.0.

If all SRD/big interactions are computed by fix srd command so what it is
required to defined that commands?

The fix srd command - what else would it be?

If particles of wall are not defined as big particles and just polymer is
defined as big particles, how i can compute interaction between srd/wall
particles such that wall particles are not in big group and i think no
longer interaction between srd/wall particles are computed by srd command so
what should i do?

We've been over this several times. The wall particles should
be "big" particles in the "big" group used when you specify the
fix srd command. Then the SRD particles will interact with the
wall particles.

If i have two kind of big particles. One is a polymer that is moving in background solvent and another one is a frozen wall
how can fix srd command distinguish between two these kind of big particles on the type of interaction?
Interactions between srd/frozen wall particles should be repulsive the same as type of interaction that is computed by fix wall/srd comannd between srd particles and srd wall
but type of interactions between polymer/srd particles is different
Thanks

how can fix srd command distinguish between two these kind of big particles
on the type of interaction?

It can't, but why should it? For an SRD particle, they are both
just big particles and they bounce off them. The fact that
they are frozen doesn't matter.

Interactions between srd/frozen wall particles should be repulsive the same
as type of interaction that is computed by fix wall/srd comannd between srd
particles and srd wall
but type of interactions between polymer/srd particles is different

How or why should they be different? A bounce is a bounce.

Steve