# boundary command

## Syntax

``` LAMMPS
boundary x y z
```

-   x,y,z = *p* or *s* or *f* or *m*, one or two letters

        *p* is periodic
        *f* is non-periodic and fixed
        *s* is non-periodic and shrink-wrapped
        *m* is non-periodic and shrink-wrapped with a minimum value

## Examples

``` LAMMPS
boundary p p f
boundary p fs p
boundary s f fm
```

## Description

Set the style of boundaries for the global simulation box in each
dimension. A single letter assigns the same style to both the lower and
upper face of the box. Two letters assigns the first style to the lower
face and the second style to the upper face. The initial size of the
simulation box is set by the [read_data](read_data),
[read_restart](read_restart), or [create_box](create_box) commands.

The style *p* means the box is periodic, so that particles interact
across the boundary, and they can exit one end of the box and re-enter
the other end. A periodic dimension can change in size due to constant
pressure boundary conditions or box deformation (see the [fix
npt](fix_nh) and [fix deform](fix_deform) commands). The *p* style must
be applied to both faces of a dimension.

The styles *f*, *s*, and *m* mean the box is non-periodic, so that
particles do not interact across the boundary and do not move from one
side of the box to the other.

For style *f*, the position of the face is fixed. If an atom moves
outside the face it will be deleted on the next timestep that
reneighboring occurs. This will typically generate an error unless you
have set the [thermo_modify lost](thermo_modify) option to allow for
lost atoms.

For style *s*, the position of the face is set so as to encompass the
atoms in that dimension (shrink-wrapping), no matter how far they move.
Note that when the difference between the current box dimensions and the
shrink-wrap box dimensions is large, this can lead to lost atoms at the
beginning of a run when running in parallel. This is due to the large
change in the (global) box dimensions also causing significant changes
in the individual subdomain sizes. If these changes are farther than the
communication cutoff, atoms will be lost. This is best addressed by
setting initial box dimensions to match the shrink-wrapped dimensions
more closely, by using *m* style boundaries (see below).

For style *m*, shrink-wrapping occurs, but is bounded by the value
specified in the data or restart file or set by the
[create_box](create_box) command. For example, if the upper z face has a
value of 50.0 in the data file, the face will always be positioned at
50.0 or above, even if the maximum z-extent of all the atoms becomes
less than 50.0. This can be useful if you start a simulation with an
empty box or if you wish to leave room on one side of the box, e.g. for
atoms to evaporate from a surface.

For triclinic (non-orthogonal) simulation boxes, if the second dimension
of a tilt factor (e.g. y for xy) is periodic, then the periodicity is
enforced with the tilt factor offset. If the first dimension is
shrink-wrapped, then the shrink wrapping is applied to the tilted box
face, to encompass the atoms. E.g. for a positive xy tilt, the xlo and
xhi faces of the box are planes tilting in the +y direction as y
increases. These tilted planes are shrink-wrapped around the atoms to
determine the x extent of the box.

See the [Howto triclinic](Howto_triclinic) page for a geometric
description of triclinic boxes, as defined by LAMMPS, and how to
transform these parameters to and from other commonly used triclinic
representations.

## Restrictions

This command cannot be used after the simulation box is defined by a
[read_data](read_data) or [create_box](create_box) command or
[read_restart](read_restart) command. See the [change_box](change_box)
command for how to change the simulation box boundaries after it has
been defined.

For 2d simulations, the z dimension must be periodic.

## Related commands

See the [thermo_modify](thermo_modify) command for a discussion of lost
atoms.

## Default

``` LAMMPS
boundary p p p
```