Dear all,
The following is an input script for simulating a collision.
units metal
boundary p p p
atom_style atomic
lattice fcc 3.615
region box block 0 4 -4 4 0 4
create_box 1 box
create_atoms 1 box
mass 1 63.546
pair_style eam
pair_coeff * * Cu_u3.eam
neighbor 1.0 bin
neigh_modify every 1 delay 5 check yes
region rgu block INF INF 2.0 INF INF INF
region rgl block INF INF INF -2.01 INF INF
group rl region rgl
group ru region rgu
region lower block INF INF -2 -0.01 INF INF
region upper block INF INF 0.0 1.99 INF INF
group lower region lower
group upper region upper
group lower2 id == 245
compute n1 all centro/atom fcc
compute n2 lower2 temp
fix 1 all nve
fix 3 lower2 temp/rescale 10 30.0 30.0 5.0 1.0
fix_modify 3 temp n2
velocity lower2 create 30 546766 temp n2
timestep 0.001
thermo 10
thermo_modify temp n2
run 0
velocity lower2 set 0.0 50.0 0.0
dump 2 all cfg 100 cu.*.cfg id type xs ys zs c_n1
run 10000

An error has occurred after running.
Created 512 atoms
128 atoms in group rl
128 atoms in group ru
128 atoms in group lower
128 atoms in group upper
1 atoms in group lower2
ERROR: Attempting to rescale a 0.0 temperature (velocity.cpp:637)

Dear all,
The following is an input script for simulating a collision.
units
metal
boundary p p
p
atom_style atomic
lattice fcc
3.615
region box block 0 4 -4 4 0
4
create_box 1
box
create_atoms 1 box
mass 1 63.546
pair_style
eam
pair_coeff * * Cu_u3.eam
neighbor 1.0 bin
neigh_modify every 1 delay 5 check yes
region rgu block INF INF 2.0 INF INF INF
region rgl block INF INF INF -2.01 INF
INF
group rl region rgl
group ru region rgu
region lower block INF INF -2 -0.01 INF INF
region upper block INF INF 0.0 1.99 INF INF
group lower region lower
group upper region upper
group lower2 id == 245
compute n1 all centro/atom fcc
compute n2 lower2 temp
fix 1 all nve
fix 3 lower2 temp/rescale 10 30.0 30.0 5.0 1.0
fix_modify 3 temp n2
velocity lower2 create 30 546766 temp n2
timestep 0.001
thermo 10
thermo_modify temp n2
run 0
velocity lower2 set 0.0 50.0 0.0
dump 2 all cfg 100 cu.*.cfg id type xs ys zs c_n1
run 10000

An error has occurred after running.
Created 512 atoms
128 atoms in group rl
128 atoms in group ru
128 atoms in group lower
128 atoms in group upper
1 atoms in group lower2
ERROR: Attempting to rescale a 0.0 temperature (velocity.cpp:637)

Any suggestions are welcome.

first you should follow the suggestions that the LAMMPS
documentation gives for debugging your own input files.

[...]
group lower2 id == 245
1 atoms in group lower2
compute n1 all centro/atom fcc
compute n2 lower2 temp
fix 1 all nve
fix 3 lower2 temp/rescale 10 30.0 30.0 5.0 1.0
fix_modify 3 temp n2
velocity lower2 create 30 546766 temp n2
ERROR: Attempting to rescale a 0.0 temperature (velocity.cpp:637)

so the velocity command is the culprit.

now to see *why* this fails, we have to step
back a little bit and we see that the compute n2
is defined for a group with a single atom. since
all temperature computes remove translational
degrees of freedom by default, this group will
always have a temperature of 0.0 unless corrected
with compute_modify. for details, please check
out the details in the manual.

one final comment. don't you think that assigning
a "temperature" from a random distribution to a
single atom is a bit over the top?