[lammps-users] warning about fene bond

dear lammps users

i am simulating a simple linear polymer. i always got this warning until the end of simulation and after some initial steps:
WARNING: FENE bond too long: 652969 15 16 1.9934636202644143 (…/bond_fene.cpp:88)

i would like to know what is wrong with my code and how to solve it. please help me to understand the problem.
thanks a lot.
the following is my code:

Linear polymer chain


units lj
atom_style bond
dimension 2
boundary p p p
special_bonds fene

Atom definition

read_data data.polymer

neighbor 0.5 bin
neigh_modify delay 0 every 1 check yes

Inter-particle potential

bond_style fene
bond_coeff 1 30.0 1.8 1.0 1.0
pair_style lj/cut 1.122
pair_modify shift yes
pair_coeff 1 1 1.0 1.0
mass 1 1.0

fix 1 all enforce2d

thermo 20
minimize 1.0e-6 0.001 1000 10000

fix 2 all langevin 1.0 1.0 1.0 7656576
fix 3 all nve

thermo 1000
dump 1 all xyz 100 Dump.xyz
run 10000000

as the warning says, your bond is stretched too much. if you look at the functional form of the FENE bond, you can see that the bond is not supposed to be longer than R_0, which is 1.8 sigma in your case. if the bond stretches a bit more, the force term will diverge and then LAMMPS will terminate.
that usually means that your parameters are not chosen well. unlike harmonic bonds, FENE bonds cannot be stretched over arbitrary distances…