# pair_style body/nparticle command

## Syntax

``` LAMMPS
pair_style body/nparticle cutoff
```

cutoff = global cutoff for interactions (distance units)

## Examples

``` LAMMPS
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](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](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
::: title
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

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

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](pair_coeff) command as in the
examples above, or in the data file or restart files read by the
[read_data](read_data) or [read_restart](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](pair_modify) command for
details.

This pair style does not support the [pair_modify](pair_modify) shift,
table, and tail options.

This pair style does not write its information to [binary restart
files](restart).

This pair style can only be used via the *pair* keyword of the
[run_style respa](run_style) 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](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](atom_style) command.

## Related commands

[pair_coeff](pair_coeff), [fix rigid](fix_rigid)

## Default

none