# [lammps-users] group/group for colloids

Dear lammps users

My system consists of 2 colloids in a LJ liquid. I can deduce the forces on one colloid by using the group/group command (see below) by summing the forces comming from the second colloid and the ones origin from the liquid, or by reading the total forces from the dump file. However, if one compares the forces in z direction in the log.file colloid/colloid= -12118 plus colloid/fluid -358 with the total forces in the dump = -12131, one sees that they are not the same. If one uses rigid atomistic particles instead of colloid ones both values coincide. What is the reason for that? Could there be a bug in the group/group command using colloid potentials?

Best regards
Sabine

pair_style colloid 15.0
compute 1 colloid1 group/group colloid2
compute 2 colloid1 group/group liquid

# time integration

fix 1 liquid nvt temp 298.0 298.0 100 drag 0.2

thermo 1
thermo_style custom step c_1 c_1[1] c_1[2] c_1[3] c_2 c_2[1] c_2[2] c_2[3]

log.lammps:

1517 1460.8907 0 0 -12118.057 -869.2625 -1886.5785 -1425.3157 -358.51072

dump_file:

dump 1 colloid1 custom 1 dump_forces_colloid id type fx fy fz

ITEM: TIMESTEP
1517
ITEM: NUMBER OF ATOMS
1
ITEM: BOX BOUNDS
0 133.11
0 133.11
0 197.2
ITEM: ATOMS id type fx fy fz
105134 2 -80.8678 -61.1997 -12131.8

I don't know. I think you are saying that the colloid1 and 2 groups
are just a single particle? I would debug this by using the
compute pair/local command and dumping out all pairwise forces
one by one, then summing them by hand. You could also put
print statements in the pair_colloid.cpp routine and see if the
answer in the compute() routine is the same as the one
in the single() routine. It sounds like you only have one
colloid-colloid interaction, so this should be easy to check.

Steve