create_box N region-ID keyword value ...
N = # of atom types to use in this simulation
region-ID = ID of region to use as simulation domain
zero or more keyword/value pairs may be appended
keyword = bond/types or angle/types or dihedral/types or improper/types or extra/bond/per/atom or extra/angle/per/atom or extra/dihedral/per/atom or extra/improper/per/atom
bond/types value = # of bond types angle/types value = # of angle types dihedral/types value = # of dihedral types improper/types value = # of improper types extra/bond/per/atom value = # of bonds per atom extra/angle/per/atom value = # of angles per atom extra/dihedral/per/atom value = # of dihedrals per atom extra/improper/per/atom value = # of impropers per atom extra/special/per/atom value = # of special neighbors per atom
create_box 2 mybox create_box 2 mybox bond/types 2 extra/bond/per/atom 1
This command creates a simulation box based on the specified region. Thus a region command must first be used to define a geometric domain. It also partitions the simulation box into a regular 3d grid of rectangular bricks, one per processor, based on the number of processors being used and the settings of the processors command. The partitioning can later be changed by the balance or fix balance commands.
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 LAMMPS 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, LAMMPS 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.
By default, 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. If you wish to define a box with tilt factors that exceed these limits, you can use the box tilt command, with a setting of large; a setting of small is the default.
See Section 6.12 of the doc pages 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.
When a prism region is used, the simulation domain should normally be periodic in the dimension that the tilt is applied to, which is given by the second dimension of the tilt factor (e.g. y for xy tilt). This is so that pairs of atoms interacting across that boundary will have one of them shifted by the tilt factor. Periodicity is set by the boundary command. For example, if the xy tilt factor is non-zero, then the y dimension should be periodic. Similarly, the z dimension should be periodic if xz or yz is non-zero. LAMMPS does not require this periodicity, but you may lose atoms if this is not the case.
Also note that if your simulation will tilt the box, e.g. via the fix deform command, the simulation box must be setup to be triclinic, even if the tilt factors are initially 0.0. You can also change an orthogonal box to a triclinic box or vice versa by using the change box command with its ortho and triclinic options.
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 as described above, LAMMPS 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 LAMMPS shrink-wraps the box around the atoms.
The optional keywords can be used to create a system that allows for bond (angle, dihedral, improper) interactions, or for molecules with special 1-2,1-3,1-4 neighbors to be added later. These optional keywords serve the same purpose as the analogous keywords that can be used in a data file which are recognized by the read_data command when it sets up a system.
Note that if these keywords are not used, then the create_box command creates an atomic (non-molecular) simulation that does not allow bonds between pairs of atoms to be defined, or a bond potential to be specified, or for molecules with special neighbors to be added to the system by commands such as create_atoms mol, fix deposit or fix pour.
As an example, see the examples/deposit/in.deposit.molecule script, which deposits molecules onto a substrate. Initially there are no molecules in the system, but they are added later by the fix deposit command. The create_box command in the script uses the bond/types and extra/bond/per/atom keywords to allow this. If the added molecule contained more than 1 special bond (allowed by default), an extra/special/per/atom keyword would also need to be specified.