[lammps-users] Problem with attractive potential

Hi all,

In a 2 chain LJ system, if the interactions are 'all' repulsive
(cut-off 2^(1/6), then the system works fine. But when I try to use
attractive cutoff for 1-1 and 2-2 but repulsive cut-off for 1-2, it
says "FENE bond too long". I have equilibrated the system first with
Soft and then with repulsive LJ for 20 million steps. Can you tell me
what wrong I am doing here?


If the model you have defined is pulling apart a FENE bond
to where it blows up (around 1.5 sigma), then there
is nothing LAMMPS can do but complain.


Even though you equilibrated for a long time with a repulsive potential, it sounds like you are suddenly turning on the attractions in one step. I can think of 3 ways to be gentler on the system, in order of which I would try first:

  1. Artificially rescale your velocities at each step to keep the beads from moving too fast, using something like this:
    fix temp all temp/rescale 1 0.0 1.0 0.05 1.0
    run 1000
    unfix temp
    I use this method successfully in a somewhat similar situation.
  2. Increasing the cutoff bit by bit and running a few hundred steps in between.
  3. Increase epsilon in small increments until you get the value you want (this would make your repulsion softer too in the beginning so you might want to mess around with it by overlaying potentials).

If all fail, I would think about a smaller timestep while still doing one or more of the above.


Hi Lisa and Michael,

Great suggestions, thanks a lot! Let me try to implement it.