"WARNING: FENE bond too long" when bond length is below the FENE cutoff length

LAMMPS LAMMPS Mailing List Mirror
1

Hello, I am using LAMMPS-7Aug19. I am attempting to simulate a piece of string by bonding particles together in a line. I want this string to be finitely extensible under any reasonable tensile force, so the FENE bond potential is appealing, but I am having trouble getting it to work. In the following code, I initialize the particles such that the nearest neighbor distance is 0.25m. I set the FENE cutoff distance R0 to be 0.255m, and I choose sigma to be such that the minimum of the FENE potential, as detailed in the LAMMPS documentation here (https://lammps.sandia.gov/doc/bond_fene.html), is very close to 0.25m. However, LAMMPS throws the error “WARNING: FENE bond too long” on the 0th timestep, even though according to the code and the dump file which I output, the bonded atoms are 0.25m from each-other, which is below the FENE cutoff distance, and continues to throw the error if I run the simulation for longer. Here is the code:

atom_style hybrid sphere bond

units si
processors * * 1
comm_modify vel yes

2

i think you are not fully understanding the implications of the value of r0.
this is a hard limit and the bond must never cross it.
with r0 of 0.255 and r of 0.25 you are within 2% of that limit. hence the warning.
with typical thermal fluctuations it is quite likely that bonds vary more than 2% unless the potential is very very steep (and then you need a tiny timestep).

axel