\(\renewcommand{\AA}{\text{Å}}\)

# pair_style body/nparticle command

## Syntax

    pair_style body/nparticle cutoff

cutoff = global cutoff for interactions (distance units)

## Examples

    pair_style body/nparticle 3.0
    pair_coeff * * 1.0 1.0
    pair_coeff 1 1 1.0 1.5 2.5

## Description

Style body/nparticle is for use with body particles and calculates pairwise body/body interactions as well as interactions between body and point-particles. See the Howto body doc page for more details on using body particles.

This pair style is designed for use with the “nparticle” body style, which is specified as an argument to the “atom-style body” command. See the Howto body page for more details about the body styles LAMMPS supports. The “nparticle” style treats a body particle as a rigid body composed of N sub-particles.

The coordinates of a body particle are its center-of-mass (COM). If the COMs of a pair of body particles are within the cutoff (global or type-specific, as specified above), then all interactions between pairs of sub-particles in the two body particles are computed. E.g. if the first body particle has 3 sub-particles, and the second has 10, then 30 interactions are computed and summed to yield the total force and torque on each body particle.

Note

In the example just described, all 30 interactions are computed even if the distance between a particular pair of sub-particles is greater than the cutoff. Likewise, no interaction between two body particles is computed if the two COMs are further apart than the cutoff, even if the distance between some pairs of their sub-particles is within the cutoff. Thus care should be used in defining the cutoff distances for body particles, depending on their shape and size.

Similar rules apply for a body particle interacting with a point particle. The distance between the two particles is calculated using the COM of the body particle and the position of the point particle. If the distance is within the cutoff and the body particle has N sub-particles, then N interactions with the point particle are computed and summed. If the distance is not within the cutoff, no interactions between the body and point particle are computed.

The interaction between two sub-particles, or a sub-particle and point particle, or between two point particles is computed as a Lennard-Jones interaction, using the standard formula

\[\begin{split}E & = 4 \epsilon \left[ \left(\frac{\sigma}{r}\right)^{12} - \left(\frac{\sigma}{r}\right)^6 \right] \qquad r < R_c \\\end{split}\]

where \(R_c\) is the cutoff. As explained above, an interaction involving one or two body sub-particles may be computed even for \(r > R_c\).

For style body, the following coefficients must be defined for each pair of atoms types via the pair_coeff command as in the examples above, or in the data file or restart files read by the read_data or read_restart commands:

- \(\epsilon\) (energy units)

- \(\sigma\) (distance units)

- cutoff (distance units)

The last coefficient is optional. If not specified, the global cutoff is used.

## Mixing, shift, table, tail correction, restart, rRESPA info

For atom type pairs I,J and I != J, the epsilon and sigma coefficients and cutoff distance for all of this pair style can be mixed. The default mix value is geometric. See the pair_modify command for details.

This pair style does not support the pair_modify shift, table, and tail options.

This pair style does not write its information to binary restart files.

This pair style can only be used via the pair keyword of the run_style respa command. It does not support the inner, middle, outer keywords.

## Restrictions

This style is part of the BODY package. It is only enabled if LAMMPS was built with that package. See the Build package page for more info.

Defining particles to be bodies so they participate in body/body or body/particle interactions requires the use of the atom_style body command.

## Related commands

pair_coeff, fix rigid

## Default

none