# How the excluded volume interaction works in atom_style 'bond'

Dear Users,
In my simulation, I am using atom_style ‘bond’ for bead-spring polymer system where I have excluded volume interactions between the beads defined as either Lennard-Jones or WCA. From standard statistical mechanics text books, it is evident that in WCA potential the particles are considered as hard spheres and Lennard-Jones cut off means two spheres can not cross each other (i.e., excluded volume interactions). As per LAMMPS documentation atom_style ‘bond’ will consider the polymer as point particles, then how this cut off for the excluded volume interactions works in the above case?

-Ligesh

Dear Users,
In my simulation, I am using atom_style 'bond' for bead-spring polymer system where I have excluded volume interactions between the beads defined as either Lennard-Jones or WCA. From standard statistical mechanics text books, it is evident that in WCA potential the particles are considered as hard spheres and Lennard-Jones cut off means two spheres can not cross each other (i.e., excluded volume interactions).

no, that is incorrect. WCA only means, no attraction. particles *can*
overlap, i.e. be closer than sigma. hard spheres would be the result
of epsilon -> oo
the lennard-jones cutoff in the case of WCA is just chosen, that you
have a repulsive-only potential.

As per LAMMPS documentation atom_style 'bond' will consider the polymer as point particles, then how this cut off for the excluded volume interactions works in the above case?

for any bonded interactions, the exact behavior depends on the value
of the special_bonds settings. the default setting is 0.0 0.0 0.0,
which means, that for any given particle, the non-bonded interactions
are not computed for atoms directly bonded to an atom (1-2 pairs),
atoms bonded to those bonded atoms (1-3 pairs), and atoms bonded to
the bonded atoms of bonded atoms (1-4 pairs). for directly bonded
atoms, you have the interactions only of the bond potentials. for
bead-spring polymers it is more common to use special_bonds lj/coul
0.0 1.0 1.0. in that case 1-3, and 1-4 pairs from the bond topology
are included in the non-bonded interactions.

axel.