# fix append/atoms command

## Syntax

``` LAMMPS
fix ID group-ID append/atoms face ... keyword value ...
```

-   ID, group-ID are documented in [fix](fix) command

-   append/atoms = style name of this fix command

-   face = *zhi*

-   zero or more keyword/value pairs may be appended

-   keyword = *basis* or *size* or *freq* or *temp* or *random* or
    *units*

        *basis* values = M itype
          M = which basis atom
          itype = atom type (1-N) to assign to this basis atom
        *size* args = Lz
          Lz = z size of lattice region appended in a single event(distance units)
        *freq* args = freq
          freq = the number of timesteps between append events
        *temp* args = target damp seed extent
          target = target temperature for the region between zhi-extent and zhi (temperature units)
          damp = damping parameter (time units)
          seed = random number seed for langevin kicks
          extent = extent of thermostatted region (distance units)
        *random* args = xmax ymax zmax seed
          *xmax*, *ymax*, *zmax* = maximum displacement in particular direction (distance units)
          *seed* = random number seed for random displacement
        *units* value = *lattice* or *box*
          *lattice* = the wall position is defined in lattice units
          *box* = the wall position is defined in simulation box units

## Examples

``` LAMMPS
fix 1 all append/atoms zhi size 5.0 freq 295 units lattice
fix 4 all append/atoms zhi size 15.0 freq 5 units box
fix A all append/atoms zhi size 1.0 freq 1000 units lattice
```

## Description

This fix creates atoms on a lattice, appended on the zhi edge of the
system box. This can be useful when a shock or wave is propagating from
zlo. This allows the system to grow with time to accommodate an
expanding wave. A simulation box must already exist, which is typically
created via the [create_box](create_box) command. Before using this
command, a lattice must also be defined using the [lattice](lattice)
command.

This fix will automatically freeze atoms on the zhi edge of the system,
so that overlaps are avoided when new atoms are appended.

The *basis* keyword specifies an atom type that will be assigned to
specific basis atoms as they are created. See the [lattice](lattice)
command for specifics on how basis atoms are defined for the unit cell
of the lattice. By default, all created atoms are assigned type = 1
unless this keyword specifies differently.

The *size* keyword defines the size in $z$ of the chunk of material to
be added.

The *random* keyword will give the atoms random displacements around
their lattice points to simulate some initial temperature.

The *temp* keyword will cause a region to be thermostatted with a
Langevin thermostat on the zhi boundary. The size of the region is
measured from zhi and is set with the *extent* argument.

The *units* keyword determines the meaning of the distance units used to
define a wall position, but only when a numeric constant is used. A
*box* value selects standard distance units as defined by the
[units](units) command (e.g., $\AA$ 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 spacings.

------------------------------------------------------------------------

## Restart, fix_modify, output, run start/stop, minimize info

No information about this fix is written to [binary restart
files](restart). None of the [fix_modify](fix_modify) options are
relevant to this fix. No global or per-atom quantities are stored by
this fix for access by various [output commands](Howto_output). No
parameter of this fix can be used with the *start/stop* keywords of the
[run](run) command. This fix is not invoked during [energy
minimization](minimize).

## Restrictions

This fix style is part of the SHOCK package. It is only enabled if
LAMMPS was built with that package. See the [Build
package](Build_package) page for more info.

The boundary on which atoms are added with append/atoms must be
shrink/minimum. The opposite boundary may be any boundary type other
than periodic.

## Related commands

[fix wall/piston](fix_wall_piston) command

## Default

The keyword defaults are size = 0.0, freq = 0, units = lattice. All
added atoms are of type 1 unless the basis keyword is used.