replicate command — LAMMPS documentation

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# replicate command

## Syntax

    replicate nx ny nz keyword ...

nx,ny,nz = replication factors in each dimension

- zero or more keywords may be appended

- keyword = bbox or bond/periodic

      bbox = use a bounding-box algorithm which is faster for large proc counts
      bond/periodic = use an algorithm that correctly replicates periodic bond loops

## Examples

For examples of replicating simple linear polymer chains (periodic or non-periodic) or periodic carbon nanotubes, see examples/replicate.

    replicate 2 3 2
    replicate 2 3 2 bbox
    replicate 2 3 2 bond/periodic

## Description

Replicate the current system 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 size of the simulation box in each dimension. A replication factor of 1 leaves the simulation domain unchanged in that dimension.

When the new simulation box is created it is 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 command. The partitioning can be changed by subsequent balance or fix balance commands.

All properties of each atom are replicated (except per-atom fix data, see the Restrictions section below). This includes their velocities, which may or may not be desirable. New atom IDs are assigned to new atoms, as are new molecule IDs. Bonds and other topology interactions are created between pairs of new atoms as well as between old and new atoms.

Note

The bond discussion which follows only refers to models with permanent covalent bonds typically defined in LAMMPS via a data file. It is not relevant to systems modeled with many-body potentials which can define bonds on-the-fly, based on the current positions of nearby atoms, e.g. models using the AIREBO or ReaxFF potentials.

If the bond/periodic keyword is not specified, bond replication is done by using the image flag for each atom to “unwrap” it out of the periodic box before replicating it. After replication is performed, atoms outside the new periodic box are wrapped back into it. This assigns correct images flags to all atoms in the system. For this to work, all original atoms in the original simulation box must have consistent image flags. This means that if two atoms have a bond between them which crosses a periodic boundary, their respective image flags will differ by 1 in that dimension.

Image flag consistency is not possible if a system has a periodic bond loop, meaning there is a chain of bonds which crosses an entire dimension and re-connects to itself across a periodic boundary. In this case you MUST use the bond/periodic keyword to correctly replicate the system. This option zeroes the image flags for all atoms and uses a different algorithm to find new (nearby) bond neighbors in the replicated system. In the final replicated system all image flags are zero (in each dimension).

Note

LAMMPS does not check for image flag consistency before performing the replication (it does issue a warning about this before a simulation is run). If the original image flags are inconsistent, the replicated system will also have inconsistent image flags, but will otherwise be correctly replicated. This is NOT the case if there is a periodic bond loop. See the next note.

Note

LAMMPS does not check for periodic bond loops. If you use the bond/periodic keyword for a system without periodic bond loops, the system will be correctly replicated, but image flag information will be lost (which may or may not be important to your model). If you do not use the bond/periodic keyword for a system with periodic bond loops, the replicated system will have invalid bonds (typically very long), resulting in bad dynamics.

If possible, the bbox keyword should be used when running on a large number of processors, as it can result in a substantial speed-up for the replication operation. It uses a bounding box to only check atoms in replicas that overlap with each processor’s new subdomain when assigning atoms to processors. It also preserves image flag information. The only drawback to the bbox option is that it requires a temporary use of more memory. Each processor must be able to store all atoms (and their per-atom data) in the original system, before it is replicated.

Note

The algorithm used by the bond/periodic keyword builds on the algorithm used by the bbox keyword and thus has the same memory requirements. If you specify only the bond/peridoic keyword it will internally set the bbox keyword as well.

## 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 generate atoms nearly on top of each other.

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 two options are possible. (1) Fixes which use the stored data in the restart file can be defined before replication and then deleted via the unfix command and re-defined after it. Or (2) the restart file can be converted to a data file (which deletes the stored fix information) and fixes defined after the replicate command. In both these scenarios, the per-atom fix information in the restart file is lost.

## Related commands

none

## Default

No settings for using the bbox or bond/periodic algorithms.