# compute com/chunk command

## Syntax

``` LAMMPS
compute ID group-ID com/chunk chunkID
```

-   ID, group-ID are documented in [compute](compute) command
-   com/chunk = style name of this compute command
-   chunkID = ID of [compute chunk/atom](compute_chunk_atom) command

## Examples

``` LAMMPS
compute 1 fluid com/chunk molchunk
```

## Description

Define a computation that calculates the center-of-mass for multiple
chunks of atoms.

In LAMMPS, chunks are collections of atoms defined by a [compute
chunk/atom](compute_chunk_atom) command, which assigns each atom to a
single chunk (or no chunk). The ID for this command is specified as
chunkID. For example, a single chunk could be the atoms in a molecule or
atoms in a spatial bin. See the [compute chunk/atom](compute_chunk_atom)
and [Howto chunk](Howto_chunk) doc pages for details of how chunks can
be defined and examples of how they can be used to measure properties of
a system.

This compute calculates the $(x,y,z)$ coordinates of the center of mass
for each chunk, which includes all effects due to atoms passing through
periodic boundaries.

Note that only atoms in the specified group contribute to the
calculation. The [compute chunk/atom](compute_chunk_atom) command
defines its own group; atoms will have a chunk ID = 0 if they are not in
that group, signifying they are not assigned to a chunk, and will thus
also not contribute to this calculation. You can specify the \"all\"
group for this command if you simply want to include atoms with non-zero
chunk IDs.

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

The coordinates of an atom contribute to the chunk\'s center-of-mass in
\"unwrapped\" form, by using the image flags associated with each atom.
See the [dump custom](dump) command for a discussion of \"unwrapped\"
coordinates. See the Atoms section of the [read_data](read_data) command
for a discussion of image flags and how they are set for each atom. You
can reset the image flags (e.g., to 0) before invoking this compute by
using the [set image](set) command.
::::

The simplest way to output the results of the compute com/chunk
calculation to a file is to use the [fix ave/time](fix_ave_time)
command, for example:

``` LAMMPS
compute cc1 all chunk/atom molecule
compute myChunk all com/chunk cc1
fix 1 all ave/time 100 1 100 c_myChunk[*] file tmp.out mode vector
```

## Output info

This compute calculates a global array where the number of rows = the
number of chunks *Nchunk* as calculated by the specified [compute
chunk/atom](compute_chunk_atom) command. The number of columns is 3 for
the $(x,y,z)$ center-of-mass coordinates of each chunk. These values can
be accessed by any command that uses global array values from a compute
as input. See the [Howto output](Howto_output) doc page for an overview
of LAMMPS output options.

The array values are \"intensive\". The array values will be in distance
[units](units).

## Restrictions

> none

## Related commands

[compute com](compute_com)

## Default

none