[lammps-users] fix bond/swap in a non-cubic box

Dear LAMMPS users and Developers,

I recently encountered a problem when using fix bond/swap. I searched the mailing list archives, and I could not find a solution. I have been using bond/swap to equilibrate long-chain polymer melts in a cubic box without any problem. For some reason, I would like to run simulations of long-chain polymer melts in a non-cubic box, where there is an angle potential with the cosine form, in addition to the FENE bond and LJ pair potentials. In this case, the program reported an error soon after it was started, such as:

ERROR on proc 10: Angle atoms 7695509 7695510 7695511 missing on proc 10 at step 103215 (../ntopo_angle_all.cpp:68)

The error seems to be related to the angle potential when running on multiple processors.

For your information, I used the LAMMPS version released on 12 Dec 2018, and I attach the following lines from the output:

orthogonal box = (-167.577 -167.577 -41.8942) to (167.577 167.577 41.8942)
8 by 12 by 4 MPI processor grid

Since the box dimension in the z direction is shorter, I tried to set the processor grid using the command "processors * * 1". But it didn't help.
Any help would be greatly appreciated. Thank you.



The missing atom error message is usually due to bad dynamics, meaning that atoms
within a 3-body angle have moved so far that they cannot be found when neighbor
lists are rebuilt (and a list of angles is tabulated).

From your description that is likely happening due to the added angle potential.
It could be that when a bond is swapped, it creates a new angle that is very far
from equilibrium, leading to huge forces and bad dynamics.