[lammps-users] question about fix bond/swap

I have a quick question about fix bond/swap that I hope someone can answer. I am doing simulations using a united atom model of polystyrene, and I want to speed up the equilibration. I used the methodology mentioned on the fix bond/swap page, numbering the molecule id for each monomer unit 1, 2, 3 … 3, 2, 1. Do I need to worry about the swap occurring between atoms in the ring and will this join two rings together? If this is the case, do I need to create a group for the backbone molecules and just perform swap on this group? Thanks

What do you mean by a ring? I think of fix bond/swap being
useful for linear chain polymers. Why would you want to
swap between rings?


I am sorry, I was not clear, I want to prevent the ring swap from occurring
and I was not sure if it will occur if I use the fix on all particles or if
I specifically need to create a group that consists of only the backbone

Ben’s question about the “ring” is referring to the aromatic group that is bonded to the backbone in polystyrene. I think the idea is to avoid having the rings swapped, but instead to swap bonds on the backbone. So I think it’d be OK so long as one doesn’t include the styrene atoms as part of the group that gets called by fix bond/swap, correct?


yes, you should exclude the ring particles from the fix group,
then they will not be eligible to swap - i.e. Ahmed was correct.

From the bond/swap doc page:

For each monomer A1, its neighbors are examined to find a possible
B1 monomer. Both A1 and B1 must be in the fix group ...