# replicate command

## Syntax

    replicate nx ny nz *keyword*

nx,ny,nz = replication factors in each dimension

-   optional *keyword* = *bbox*

        *bbox* = only check atoms in replicas that overlap with a processor's subdomain

## Examples

``` LAMMPS
replicate 2 3 2
```

## Description

Replicate the current simulation one or more times in each dimension.
For example, replication factors of 2,2,2 will create a simulation with
8x as many atoms by doubling the simulation domain in each dimension. A
replication factor of 1 in a dimension leaves the simulation domain
unchanged. When the new simulation box is created it is also partitioned
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](processors) command. The partitioning can later be changed
by the [balance](balance) or [fix balance](fix_balance) commands.

All properties of the atoms are replicated, including their velocities,
which may or may not be desirable. New atom IDs are assigned to new
atoms, as are molecule IDs. Bonds and other topology interactions are
created between pairs of new atoms as well as between old and new atoms.
This is done by using the image flag for each atom to \"unwrap\" it out
of the periodic box before replicating it.

This means that any molecular bond you specify in the original data file
that crosses a periodic boundary should be between two atoms with image
flags that differ by 1. This will allow the bond to be unwrapped
appropriately.

The optional keyword *bbox* uses a bounding box to only check atoms in
replicas that overlap with a processor\'s subdomain when assigning atoms
to processors. It typically results in a substantial speedup when using
the replicate command on a large number of processors. It does require
temporary use of more memory, specifically that each processor can store
all atoms in the entire system before it is replicated.

## Restrictions

A 2d simulation cannot be replicated in the z dimension.

If a simulation is non-periodic in a dimension, care should be used when
replicating it in that dimension, as it may put atoms nearly on top of
each other.

:::: note
::: title
Note
:::

You cannot use the replicate command on a system which has a molecule
that spans the box and is bonded to itself across a periodic boundary,
so that the molecule is effectively a loop. A simple example would be a
linear polymer chain that spans the simulation box and bonds back to
itself across the periodic boundary. More realistic examples would be a
CNT (meant to be an infinitely long CNT) or a graphene sheet or a bulk
periodic crystal where there are explicit bonds specified between near
neighbors. (Note that this only applies to systems that have permanent
bonds as specified in the data file. A CNT that is just atoms modeled
with the [AIREBO potential](pair_airebo) has no such permanent bonds, so
it can be replicated.) The reason replication does not work with those
systems is that the image flag settings described above cannot be made
consistent. I.e. it is not possible to define images flags so that when
every pair of bonded atoms is unwrapped (using the image flags), they
will be close to each other. The only way the replicate command could
work in this scenario is for it to break a bond, insert more atoms, and
re-connect the loop for the larger simulation box. But it is not clever
enough to do this. So you will have to construct a larger version of
your molecule as a pre-processing step and input a new data file to
LAMMPS.
::::

If the current simulation was read in from a restart file (before a run
is performed), there must not be any fix information stored in the file
for individual atoms. Similarly, no fixes can be defined at the time the
replicate command is used that require vectors of atom information to be
stored. This is because the replicate command does not know how to
replicate that information for new atoms it creates. To work around this
restriction, restart files may be converted into data files and fixes
may be undefined via the [unfix](unfix) command before and redefined
after the replicate command.

## Related commands

none

## Default

none