.. index:: compute omega/chunk

compute omega/chunk command
===========================

Syntax
""""""

.. parsed-literal::

   compute ID group-ID omega/chunk chunkID

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

Examples
""""""""

.. parsed-literal::

   compute 1 fluid omega/chunk molchunk

Description
"""""""""""

Define a computation that calculates the angular velocity (omega) of
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 3 components of the angular velocity
vector for each chunk, via the formula L = Iw where L is the angular
momentum vector of the chunk, I is its moment of inertia tensor, and w
is omega = angular velocity of the chunk.  The calculation 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 angular
   velocity 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 omega/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 omega/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 3 xyz components of the angular velocity for 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
velocity/distance :doc:`units <units>`.

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

Related commands
""""""""""""""""

:doc:`variable omega() function <variable>`

**Default:** none


.. _lws: http://lammps.sandia.gov
.. _ld: Manual.html
.. _lc: Section_commands.html#comm