# change_box command ## Syntax ``` LAMMPS 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 ``` LAMMPS 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](boundary) command. The size and shape of the initial simulation box are specified by the [create_box](create_box) or [read_data](read_data) or [read_restart](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](fix_nh) or [fix deform](fix_deform) commands. The [create_box](create_box), [read data](read_data), and [read_restart](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 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. 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](boundary) command) which may change simulation box boundaries, and atoms are migrated to new owning processors. :::: note ::: title 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](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: :::: ``` LAMMPS 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 all boundary s s s # this will work ``` :::: note ::: title 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. :::: :::: note ::: title 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. :::: :::: note ::: title 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](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. You may wish to re-balance the atoms by using the [balance](balance) command if that is the case. :::: :::: note ::: title Note ::: You cannot use this command after reading a restart file (and before a run is performed) if the restart file stored per-atom information from a fix and any of the specified keywords change the box size or shape or boundary conditions. This is because atoms may be moved to new processors and the restart info will not migrate with them. LAMMPS will generate an error if this could happen. Only the *ortho* and *triclinic* keywords do not trigger this error. One solution is to perform a \"run 0\" command before using the change_box command. This clears the per-atom restart data, whether it has been re-assigned to a new fix or not. :::: :::: note ::: title 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 [Howto triclinic](Howto_triclinic) doc 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: ``` LAMMPS 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: ``` LAMMPS 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: ``` LAMMPS 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: ``` LAMMPS change_box all x scale 1.1 z volume y scale 1.1 z volume ``` :::: note ::: title 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](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](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](read_data) or [create_box](create_box) or [read_restart](read_restart) command. This command allows the boundary conditions to be changed later in your input script. Also note that the [read_restart](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. :::: note ::: title Note ::: Changing a periodic boundary to a non-periodic one will also cause the image flag for that dimension of all atoms to be reset to 0. LAMMPS will print a warning message, if that happens. Please note that this reset can lead to undesired changes when atoms are involved in bonded interactions that straddle periodic boundaries and thus the values of the image flag differs for those atoms. :::: ------------------------------------------------------------------------ The *ortho* and *triclinic* keywords convert the simulation box to be orthogonal or triclinic (non-orthogonal). The simulation box is defined as either orthogonal or triclinic when it is created via the [create_box](create_box), [read_data](read_data), or [read_restart](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](Howto_nemd) can be run with deformation via the [fix deform](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* command 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](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](units) command, e.g. Angstroms for units = real or metal. A *lattice* value means the distance units are in lattice spacings. The [lattice](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](dump) can be active, nor can a [fix deform](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](undump) or [unfix](unfix) command is also used to turn them off. ## Related commands [fix deform](fix_deform), [boundary](boundary) ## Default The option default is units = lattice.