# fix wall/body/polygon command

## Syntax

    fix ID group-ID wall/body/polygon k_n c_n c_t wallstyle args keyword values ...

-   ID, group-ID are documented in [fix](fix) command

-   wall/body/polygon = style name of this fix command

-   k_n = normal repulsion strength (force/distance or pressure units)

-   c_n = normal damping coefficient (force/distance or pressure units)

-   c_t = tangential damping coefficient (force/distance or pressure
    units)

-   wallstyle = *xplane* or *yplane* or *zcylinder*

-   args = list of arguments for a particular style

        *xplane* or *yplane* args = lo hi
          lo,hi = position of lower and upper plane (distance units), either can be NULL)
        *zcylinder* args = radius
          radius = cylinder radius (distance units)

-   zero or more keyword/value pairs may be appended to args

-   keyword = *wiggle*

        *wiggle* values = dim amplitude period
          dim = *x* or *y* or *z*
          amplitude = size of oscillation (distance units)
          period = time of oscillation (time units)

## Examples

``` LAMMPS
fix 1 all wall/body/polygon 1000.0 20.0 5.0 xplane -10.0 10.0
```

## Description

This fix is for use with 2d models of body particles of style
*rounded/polygon*. It bounds the simulation domain with wall(s). All
particles in the group interact with the wall when they are close enough
to touch it. The nature of the interaction between the wall and the
polygon particles is the same as that between the polygon particles
themselves, which is similar to a Hookean potential. See the [Howto
body](Howto_body) page for more details on using body particles.

The parameters *k_n*, *c_n*, *c_t* have the same meaning and units as
those specified with the [pair_style
body/rounded/polygon](pair_body_rounded_polygon) command.

The *wallstyle* can be planar or cylindrical. The 2 planar options
specify a pair of walls in a dimension. Wall positions are given by *lo*
and *hi*. Either of the values can be specified as NULL if a single wall
is desired. For a *zcylinder* wallstyle, the cylinder\'s axis is at x =
y = 0.0, and the radius of the cylinder is specified.

Optionally, the wall can be moving, if the *wiggle* keyword is appended.

For the *wiggle* keyword, the wall oscillates sinusoidally, similar to
the oscillations of particles which can be specified by the [fix
move](fix_move) command. This is useful in packing simulations of
particles. The arguments to the *wiggle* keyword specify a dimension for
the motion, as well as it\'s *amplitude* and *period*. Note that if the
dimension is in the plane of the wall, this is effectively a shearing
motion. If the dimension is perpendicular to the wall, it is more of a
shaking motion. A *zcylinder* wall can only be wiggled in the z
dimension.

Each timestep, the position of a wiggled wall in the appropriate *dim*
is set according to this equation:

    position = coord + A - A cos (omega \* delta)

where *coord* is the specified initial position of the wall, *A* is the
*amplitude*, *omega* is 2 PI / *period*, and *delta* is the time elapsed
since the fix was specified. The velocity of the wall is set to the
derivative of this expression.

## Restart, fix_modify, output, run start/stop, minimize info

None of the [fix_modify](fix_modify) options are relevant to this fix.
No global or per-atom quantities are stored by this fix for access by
various [output commands](Howto_output). No parameter of this fix can be
used with the *start/stop* keywords of the [run](run) command. This fix
is not invoked during [energy minimization](minimize).

## Restrictions

This fix 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.

Any dimension (xy) that has a wall must be non-periodic.

## Related commands

[atom_style body](atom_style), [pair_style
body/rounded/polygon](pair_body_rounded_polygon)

## Default

none