.. index:: compute com/chunk

compute com/chunk command
=========================

Syntax
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.. parsed-literal::

   compute ID group-ID com/chunk chunkID

* ID, group-ID are documented in :doc:`compute <compute>` command
* com/chunk = style name of this compute command
* chunkID = ID of :doc:`compute chunk/atom <compute_chunk_atom>` command

Examples
""""""""

.. code-block:: 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 :doc:`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 :doc:`compute chunk/atom <compute_chunk_atom>` and :doc:`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 :doc:`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::

   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 :doc:`dump custom <dump>` command
   for a discussion of "unwrapped" coordinates.  See the Atoms section of
   the :doc:`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 :doc:`set image <set>` command.

The simplest way to output the results of the compute com/chunk
calculation to a file is to use the :doc:`fix ave/time <fix_ave_time>`
command, for example:

.. code-block:: 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 :doc:`compute chunk/atom <compute_chunk_atom>` command.  The number of columns =
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 :doc:`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 :doc:`units <units>`.

Restrictions
""""""""""""
 none

Related commands
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:doc:`compute com <compute_com>`

**Default:** none
