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compute_com_chunk.txt
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.. index:: compute com/chunk
compute com/chunk command
=========================
Syntax
""""""
.. 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
""""""""
.. parsed-literal::
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>` doc page and ":ref:`Section_howto 23 <howto_23>` 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 thru
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:
.. parsed-literal::
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 :ref:`Section_howto 15 <howto_15>` 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
""""""""""""""""
:doc:`compute com <compute_com>`
**Default:** none
.. _lws: http://lammps.sandia.gov
.. _ld: Manual.html
.. _lc: Section_commands.html#comm
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