# compute vcm/chunk command

## Syntax

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

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

## Examples

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

## Description

Define a computation that calculates the center-of-mass velocity 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 $(x,y,z)$ components of the center-of-mass
velocity for each chunk. This is done by summing mass\*velocity for each
atom in the chunk and dividing the sum by the total mass of the chunk.

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.

The simplest way to output the results of the compute vcm/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 vcm/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 is 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 velocity 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) page for an
overview of LAMMPS output options.

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

## Restrictions

none

## Related commands

none

## Default

none