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# boundary command

## Syntax

    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

    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_restart, or 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 and fix deform commands). The p style must be applied to both faces of a dimension. For 2d simulations the z dimension must be periodic (which is the default).

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 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 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.

LAMMPS also allows use of triclinic (non-orthogonal) simulation boxes. See the Howto triclinic page for a description of both general and restricted triclinic boxes and how to define them. General triclinic boxes (arbitrary edge vectors A, B, and C) are converted internally to restricted triclinic boxes with tilt factors (xy,xz,yz) which skew an otherwise orthogonal box.

The boundary <boundary> command settings explained above for the 6 faces of an orthogonal box also apply in similar manner to the 6 faces of a restricted triclinic box (and thus to the corresponding 6 faces of a general triclinic box), with the following context.

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. This means that for y periodicity a particle which exits the lower y boundary is displaced in the x-direction by xy before it re-enters the upper y boundary. And vice versa if a particle exits the upper y boundary. Likewise the ghost atoms surrounding a particle near the lower y boundary include images of particles near the upper y-boundary which are displaced in the x-direction by xy. Similar rules apply for z-periodicity and the xz and/or yz tilt factors.

If the first dimension of a tilt factor 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. The position of these tilted planes are adjusted dynamically to shrink-wrap around the atoms to determine the xlo and xhi extents of the box.

## Restrictions

This command cannot be used after the simulation box is defined by a read_data or create_box command or read_restart command. See the 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 command for a discussion of lost atoms.

## Default

    boundary p p p