# compute property/chunk command

## Syntax

``` LAMMPS
compute ID group-ID property/chunk chunkID input1 input2 ...
```

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

-   property/chunk = style name of this compute command

-   chunkID = ID of [compute chunk/atom](compute_chunk_atom) command
    that defines the chunks

-   input1,etc = one or more attributes

        attributes = count, id, coord1, coord2, coord3
          count = # of atoms in chunk
          id = original chunk IDs before compression by `compute chunk/atom <compute_chunk_atom>`__
          coord123 = coordinates for spatial bins calculated by `compute chunk/atom <compute_chunk_atom>`__

## Examples

``` LAMMPS
compute 1 all property/chunk bin2d id count
compute 1 all property/chunk myChunks id coord1
```

## Description

Define a computation that stores the specified attributes of 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 and stores the specified attributes of chunks as
global data so they can be accessed by other [output
commands](Howto_output) and used in conjunction with other commands that
generate per-chunk data, such as [compute com/chunk](compute_com_chunk)
or [compute msd/chunk](compute_msd_chunk).

Note that only atoms in the specified group contribute to the
calculation of the *count* attribute. 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 *count* attribute is the number of atoms in the chunk.

The *id* attribute stores the original chunk ID for each chunk. It can
only be used if the *compress* keyword was set to *yes* for the [compute
chunk/atom](compute_chunk_atom) command referenced by chunkID. This
means that the original chunk IDs (e.g., molecule IDs) will have been
compressed to remove chunk IDs with no atoms assigned to them. Thus a
compressed chunk ID of 3 may correspond to an original chunk ID
(molecule ID in this case) of 415. The *id* attribute will then be 415
for the third chunk.

The *coordN* attributes can only be used if a *binning* style was used

in the [compute chunk/atom](compute_chunk_atom) command referenced by
chunkID. For *bin/1d*, *bin/2d*, and *bin/3d* styles the attribute is
the center point of the bin in the corresponding dimension. Style
*bin/1d* only defines a *coord1* attribute. Style *bin/2d* adds a
*coord2* attribute. Style *bin/3d* adds a *coord3* attribute.

Note that if the value of the *units* keyword used in the [compute
chunk/atom command](compute_chunk_atom) is *box* or *lattice*, the
*coordN* attributes will be in distance [units](units). If the value of
the *units* keyword is *reduced*, the *coordN* attributes will be in
unitless reduced units (0-1).

The simplest way to output the results of the compute property/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 myChunk1 all property/chunk cc1 count
compute myChunk2 all com/chunk cc1
fix 1 all ave/time 100 1 100 c_myChunk1 c_myChunk2[*] file tmp.out mode vector
```

## Output info

This compute calculates a global vector or global array depending on the
number of input values. The length of the vector or number of rows in
the array is the number of chunks.

This compute calculates a global vector or global array where the number
of rows = the number of chunks *Nchunk* as calculated by the specified
[compute chunk/atom](compute_chunk_atom) command. If a single input is
specified, a global vector is produced. If two or more inputs are
specified, a global array is produced where the number of columns = the
number of inputs. The vector or array can be accessed by any command
that uses global values from a compute as input. See the [Howto
output](Howto_output) page for an overview of LAMMPS output options.

The vector or array values are \"intensive\". The values will be
unitless or in the units discussed above.

## Restrictions

> none

## Related commands

[fix ave/chunk](fix_ave_chunk)

## Default

none