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### change_box command

Syntax:

    change_box group-ID parameter args ... keyword args ...

- group-ID = ID of group of atoms to (optionally) displace
- one or more parameter/arg pairs may be appended

      parameter = x or y or z or xy or xz or yz or boundary or ortho or triclinic or set or remap
        x, y, z args = style value(s)
          style = final or delta or scale or volume
            final values = lo hi
              lo hi = box boundaries after displacement (distance units)
            delta values = dlo dhi
              dlo dhi = change in box boundaries after displacement (distance units)
            scale values = factor
              factor = multiplicative factor for change in box length after displacement
            volume value = none = adjust this dim to preserve volume of system
        xy, xz, yz args = style value
          style = final or delta
            final value = tilt
              tilt = tilt factor after displacement (distance units)
            delta value = dtilt
              dtilt = change in tilt factor after displacement (distance units)
        boundary args = 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
        ortho args = none = change box to orthogonal
        triclinic args = none = change box to triclinic
        set args = none = store state of current box
        remap args = none = remap atom coords from last saved state to current box

- zero or more keyword/value pairs may be appended
- keyword = units

        units value = lattice or box
          lattice = distances are defined in lattice units
          box = distances are defined in simulation box units

Examples:

    change_box all xy final -2.0 z final 0.0 5.0 boundary p p f remap units box
    change_box all x scale 1.1 y volume z volume remap

Description:

Change the volume and/or shape and/or boundary conditions for the simulation box. Orthogonal simulation boxes have 3 adjustable size parameters (x,y,z). Triclinic (non-orthogonal) simulation boxes have 6 adjustable size/shape parameters (x,y,z,xy,xz,yz). Any or all of them can be adjusted independently by this command. Thus it can be used to expand or contract a box, or to apply a shear strain to a non-orthogonal box. It can also be used to change the boundary conditions for the simulation box, similar to the boundary command.

The size and shape of the initial simulation box are specified by the create_box or read_data or read_restart command used to setup the simulation. The size and shape may be altered by subsequent runs, e.g. by use of the fix npt or fix deform commands. The create_box, read data, and read_restart commands also determine whether the simulation box is orthogonal or triclinic and their doc pages explain the meaning of the xy,xz,yz tilt factors.

See Section_howto 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.

The keywords used in this command are applied sequentially to the simulation box and the atoms in it, in the order specified.

Before the sequence of keywords are invoked, the current box size/shape is stored, in case a remap keyword is used to map the atom coordinates from a previously stored box size/shape to the current one.

After all the keywords have been processed, any shrink-wrap boundary conditions are invoked (see the boundary command) which may change simulation box boundaries, and atoms are migrated to new owning processors.

IMPORTANT_NOTE: This means that you cannot use the change_box command to enlarge a shrink-wrapped box, e.g. to make room to insert more atoms via the create_atoms command, because the simulation box will be re-shrink-wrapped before the change_box command completes. Instead you could do something like this, assuming the simulation box is non-periodic and atoms extend from 0 to 20 in all dimensions:

    change_box all x final -10 20
    create_atoms 1 single -5 5 5   # this will fail to insert an atom

    change_box all x final -10 20 boundary f s s
    create_atoms 1 single -5 5 5
    change_box boundary s s s      # this will work

IMPORTANT NOTE: Unlike the earlier "displace_box" version of this command, atom remapping is NOT performed by default. This command allows remapping to be done in a more general way, exactly when you specify it (zero or more times) in the sequence of transformations. Thus if you do not use the remap keyword, atom coordinates will not be changed even if the box size/shape changes. If a uniformly strained state is desired, the remap keyword should be specified.

IMPORTANT NOTE: It is possible to lose atoms with this command. E.g. by changing the box without remapping the atoms, and having atoms end up outside of non-periodic boundaries. It is also possible to alter bonds between atoms straddling a boundary in bad ways. E.g. by converting a boundary from periodic to non-periodic. It is also possible when remapping atoms to put them (nearly) on top of each other. E.g. by converting a boundary from non-periodic to periodic. All of these will typically lead to bad dynamics and/or generate error messages.

IMPORTANT NOTE: The simulation box size/shape can be changed by arbitrarily large amounts by this command. This is not a problem, except that the mapping of processors to the simulation box is not changed from its initial 3d configuration; see the processors command. Thus, if the box size/shape changes dramatically, the mapping of processors to the simulation box may not end up as optimal as the initial mapping attempted to be.

IMPORTANT NOTE: Because the keywords used in this command are applied one at a time to the simulation box and the atoms in it, care must be taken with triclinic cells to avoid exceeding the limits on skew after each transformation in the sequence. If skew is exceeded before the final transformation this can be avoided by changing the order of the sequence, or breaking the transformation into two or more smaller transformations. For more information on the allowed limits for box skew see the discussion on triclinic boxes on this page.

For the x, y, and z parameters, this is the meaning of their styles and values.

For style final, the final lo and hi box boundaries of a dimension are specified. The values can be in lattice or box distance units. See the discussion of the units keyword below.

For style delta, plus or minus changes in the lo/hi box boundaries of a dimension are specified. The values can be in lattice or box distance units. See the discussion of the units keyword below.

For style scale, a multiplicative factor to apply to the box length of a dimension is specified. For example, if the initial box length is 10, and the factor is 1.1, then the final box length will be 11. A factor less than 1.0 means compression.

The volume style changes the specified dimension in such a way that the overall box volume remains constant with respect to the operation performed by the preceding keyword. The volume style can only be used following a keyword that changed the volume, which is any of the x, y, z keywords. If the preceding keyword "key" had a volume style, then both it and the current keyword apply to the keyword preceding "key". I.e. this sequence of keywords is allowed:

    change_box all x scale 1.1 y volume z volume

The volume style changes the associated dimension so that the overall box volume is unchanged relative to its value before the preceding keyword was invoked.

If the following command is used, then the z box length will shrink by the same 1.1 factor the x box length was increased by:

    change_box all x scale 1.1 z volume

If the following command is used, then the y,z box lengths will each shrink by sqrt(1.1) to keep the volume constant. In this case, the y,z box lengths shrink so as to keep their relative aspect ratio constant:

    change_box all"x scale 1.1 y volume z volume

If the following command is used, then the final box will be a factor of 10% larger in x and y, and a factor of 21% smaller in z, so as to keep the volume constant:

    change_box all x scale 1.1 z volume y scale 1.1 z volume

IMPORTANT NOTE: For solids or liquids, when one dimension of the box is expanded, it may be physically undesirable to hold the other 2 box lengths constant since that implies a density change. For solids, adjusting the other dimensions via the volume style may make physical sense (just as for a liquid), but may not be correct for materials and potentials whose Poisson ratio is not 0.5.

For the scale and volume styles, the box length is expanded or compressed around its mid point.

For the xy, xz, and yz parameters, this is the meaning of their styles and values. Note that changing the tilt factors of a triclinic box does not change its volume.

For style final, the final tilt factor is specified. The value can be in lattice or box distance units. See the discussion of the units keyword below.

For style delta, a plus or minus change in the tilt factor is specified. The value can be in lattice or box distance units. See the discussion of the units keyword below.

All of these styles change the xy, xz, yz tilt factors. In LAMMPS, tilt factors (xy,xz,yz) for triclinic boxes are required to be no 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 equivalent. Any tilt factor specified by this command must be within these limits.

The boundary keyword takes arguments that have exactly the same meaning as they do for the boundary command. 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 style p means the box is periodic; the other styles mean non-periodic. For style f, the position of the face is fixed. 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. For style m, shrink-wrapping occurs, but is bounded by the current box edge in that dimension, so that the box will become no smaller. See the boundary command for more explanation of these style options.

Note that the "boundary" command itself can only be used before the simulation box is defined via a read_data or create_box or read_restart command. This command allows the boundary conditions to be changed later in your input script. Also note that the read_restart will change boundary conditions to match what is stored in the restart file. So if you wish to change them, you should use the change_box command after the read_restart command.

The ortho and triclinic keywords convert the simulation box to be orthogonal or triclinic (non-orthongonal). See this section for a discussion of how non-orthongal boxes are represented in LAMMPS.

The simulation box is defined as either orthogonal or triclinic when it is created via the create_box, read_data, or read_restart commands.

These keywords allow you to toggle the existing simulation box from orthogonal to triclinic and vice versa. For example, an initial equilibration simulation can be run in an orthogonal box, the box can be toggled to triclinic, and then a non-equilibrium MD (NEMD) simulation can be run with deformation via the fix deform command.

If the simulation box is currently triclinic and has non-zero tilt in xy, yz, or xz, then it cannot be converted to an orthogonal box.

The set keyword saves the current box size/shape. This can be useful if you wish to use the remap keyword more than once or if you wish it to be applied to an intermediate box size/shape in a sequence of keyword operations. Note that the box size/shape is saved before any of the keywords are processed, i.e. the box size/shape at the time the create_box command is encountered in the input script.

The remap keyword remaps atom coordinates from the last saved box size/shape to the current box state. For example, if you stretch the box in the x dimension or tilt it in the xy plane via the x and xy keywords, then the remap commmand will dilate or tilt the atoms to conform to the new box size/shape, as if the atoms moved with the box as it deformed.

Note that this operation is performed without regard to periodic boundaries. Also, any shrink-wrapping of non-periodic boundaries (see the boundary command) occurs after all keywords, including this one, have been processed.

Only atoms in the specified group are remapped.

The units keyword determines the meaning of the distance units used to define various arguments. A box value selects standard distance units as defined by the units command, e.g. Angstroms for units = real or metal. A lattice value means the distance units are in lattice spacings. The lattice command must have been previously used to define the lattice spacing.

Restrictions:

If you use the ortho or triclinic keywords, then at the point in the input script when this command is issued, no dumps can be active, nor can a fix ave/spatial or fix deform be active. This is because these commands test whether the simulation box is orthogonal when they are first issued. Note that these commands can be used in your script before a change_box command is issued, so long as an undump or unfix command is also used to turn them off.

Related commands:

fix deform, boundary

Default:

The option default is units = lattice.