# create_box command

## Syntax

    create_box N region-ID

- N = # of atom types to use in this simulation
- region-ID = ID of region to use as simulation domain (optional)

## Examples

    create_box 2 mybox
    create_box 1

## Description

This command creates a simulation box based on a specified region. Either a region command must first be used to define a geometric domain and its region-ID must be specified as argument, or a `simulation_domain` command must be used first. In the latter case the box defined by the `simulation_domain` is automatically used if no region-ID is specified.

The argument N is the number of atom types that will be used in the simulation.

If the region is not of style prism, then LIGGGHTS(R)-PUBLIC encloses the region (block, sphere, etc) with an axis-aligned orthogonal bounding box which becomes the simulation domain.

If the region is of style prism, LIGGGHTS(R)-PUBLIC creates a non-orthogonal simulation domain shaped as a parallelepiped with triclinic symmetry. As defined by the region prism command, the parallelepiped has its “origin” at (xlo,ylo,zlo) and is defined by 3 edge vectors starting from the origin given by A = (xhi-xlo,0,0); B = (xy,yhi-ylo,0); C = (xz,yz,zhi-zlo). Xy,xz,yz can be 0.0 or positive or negative values and are called “tilt factors” because they are the amount of displacement applied to faces of an originally orthogonal box to transform it into the parallelipiped.

A prism region used with the create_box command must have tilt factors (xy,xz,yz) that do not skew the box more than half the distance of the parallel box length. For example, if xlo = 2 and xhi = 12, then the x box length is 10 and the xy tilt factor must be between -5 and 5. Similarly, both xz and yz must be between -(xhi-xlo)/2 and +(yhi-ylo)/2. Note that this is not a limitation, since if the maximum tilt factor is 5 (as in this example), then configurations with tilt = ..., -15, -5, 5, 15, 25, ... are all geometrically equivalent.

See Section_howto 12 of the doc pages for a geometric description of triclinic boxes, as defined by LIGGGHTS(R)-PUBLIC, and how to transform these parameters to and from other commonly used triclinic representations.

When a prism region is used, the simulation domain must be periodic in any dimensions with a non-zero tilt factor, as defined by the boundary command. I.e. if the xy tilt factor is non-zero, then both the x and y dimensions must be periodic. Similarly, x and z must be periodic if xz is non-zero and y and z must be periodic if yz is non-zero. Also note that if your simulation will tilt the box, e.g. via the fix deform command, the simulation box must be defined as triclinic, even if the tilt factors are initially 0.0.

Warning

If the system is non-periodic (in a dimension), then you should not make the lo/hi box dimensions (as defined in your region command) radically smaller/larger than the extent of the atoms you eventually plan to create, e.g. via the create_atoms command. For example, if your atoms extend from 0 to 50, you should not specify the box bounds as -10000 and 10000. This is because LIGGGHTS(R)-PUBLIC uses the specified box size to layout the 3d grid of processors. A huge (mostly empty) box will be sub-optimal for performance when using “fixed” boundary conditions (see the boundary command). When using “shrink-wrap” boundary conditions (see the boundary command), a huge (mostly empty) box may cause a parallel simulation to lose atoms the first time that LIGGGHTS(R)-PUBLIC shrink-wraps the box around the atoms.

## Restrictions

An atom_style and one of region or `simulation_domain` must have been previously defined to use this command.

## Related commands

create_atoms, region, `simulation_domain`

Default: none