I am looking to use LAMMPS to do an energy minimization and annealing of a polymer that contains many aromatic rings throughout its backbone. For consistency with future stages of the simulation, I would like to use the TraPPE-UA model which uses a 9 site model for benzene (6 sites for the CH groups and 3 point charges to accurately describe the quadrupole moment). While I can easily add charges to the atoms themselves, I don’t see how to create point charges that are fixed relative to the aromatic rings. Any suggestions?
One option I have considered is creating massless atoms that are bonded to the aromatic rings and held in place with some type of a fix. However, there will be over 1000 point charges, which may be too many (“In LAMMPS, only small clusters of atoms can be constrained…”). If not, what type of fix should I use?
The only way currently in LAMMPS to add extra charges at non-atom
locations would by to add massless atoms as you suggest. Exception
is TIP4P which automates this for the one extra point charge.
If the entire aromatic ring is rigid, then you could use fix rigid and define
the collections of 1000+ atoms to be rigid.