ERROR: Could not find fix group ID

LAMMPS LAMMPS Beginners
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1

Hi dear lammps users:
I want to simulate a nylon6 polymer with md and reach a suitable equilibrium state with temperature changes, when I run my system with the following input script:
units real
neigh_modify delay 2 every 1

atom_style full
bond_style harmonic
angle_style harmonic

pair_style soft 10.0
pair_modify mix arithmetic
kspace_style none

read_data Polyny6-15.data

################ lattice #######################
lattice sc 1
#################################################
log log.Soft

thermo 100
thermo_style custom step temp press etotal vol
fix Soft all nve 298.15 298.15
fix ShakeE1 water shake 1e-6 1000000 0 m 1.008 a 154
#################### dump ######################
dump dumpSoft all custom 1000 atoms.Soft.xyz id mol type q x y z
dump_modify dumpSoft sort id
restart 500000 restartSoft.*
run 1000000

I faced with this error:
ERROR: Could not find fix group ID water (…/modify.cpp:822)
Last command: fix ShakeE1 water shake 1e-6 1000000 0 m 1.008 a 154
I read lammps manual bun I can’t find my problem. I need any suggestion/advice about this errors and about something obvious mistake in my input script.

Thanks very much
Moein

2

The error message is pretty much self-explanatory.
Your fix shake command references a group “water”, but there is no group command defining that group. How should LAMMPS know which atoms to apply fix shake on?

3

Dear akohlmey; first thanks your answer.
I am a newcomer in LAMMPS and I am creating the final input script using an example input script. The system I am simulating is a nylon 6 polymer, there is no water in it. In your opinion, how should I correct this command?

Moein

4

Specifically as a beginner, if you copy commands from some other file, you need to study and understand what each single command does, what it is used for in the specific example, whether this applies to your system, and whether you need to make adjustments. LAMMPS has a very detailed documentation and reference for all commands. So please help yourself to a good and thorough read.

5

Thank you for the explanation, dear Axcel.