103 lines
3.5 KiB
ReStructuredText
103 lines
3.5 KiB
ReStructuredText
.. index:: compute omega/chunk
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compute omega/chunk command
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===========================
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Syntax
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""""""
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.. code-block:: LAMMPS
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compute ID group-ID omega/chunk chunkID
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* ID, group-ID are documented in :doc:`compute <compute>` command
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* omega/chunk = style name of this compute command
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* chunkID = ID of :doc:`compute chunk/atom <compute_chunk_atom>` command
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Examples
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""""""""
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.. code-block:: LAMMPS
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compute 1 fluid omega/chunk molchunk
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Description
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"""""""""""
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Define a computation that calculates the angular velocity (omega) of
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multiple chunks of atoms.
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In LAMMPS, chunks are collections of atoms defined by a
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:doc:`compute chunk/atom <compute_chunk_atom>` command, which assigns each atom
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to a single chunk (or no chunk). The ID for this command is specified
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as chunkID. For example, a single chunk could be the atoms in a
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molecule or atoms in a spatial bin. See the
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:doc:`compute chunk/atom <compute_chunk_atom>` and
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:doc:`Howto chunk <Howto_chunk>`
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doc pages for details of how chunks can be defined and examples of how
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they can be used to measure properties of a system.
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This compute calculates the three components of the angular velocity
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vector for each chunk via the formula
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:math:`\vec L = \mathrm{I}\cdot \vec\omega`, where :math:`\vec L` is the
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angular momentum vector of the chunk, :math:`\mathrm{I}` is its moment of
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inertia tensor, and :math:`\omega` is the angular velocity of the chunk.
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The calculation includes all effects due to atoms passing through periodic
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boundaries.
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Note that only atoms in the specified group contribute to the
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calculation. The :doc:`compute chunk/atom <compute_chunk_atom>` command
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defines its own group; atoms will have a chunk ID = 0 if they are not
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in that group, signifying they are not assigned to a chunk, and will
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thus also not contribute to this calculation. You can specify the
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"all" group for this command if you simply want to include atoms with
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non-zero chunk IDs.
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.. note::
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The coordinates of an atom contribute to the chunk's angular
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velocity in "unwrapped" form, by using the image flags associated with
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each atom. See the :doc:`dump custom <dump>` command for a discussion
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of "unwrapped" coordinates. See the Atoms section of the
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:doc:`read_data <read_data>` command for a discussion of image flags and
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how they are set for each atom. You can reset the image flags
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(e.g., to 0) before invoking this compute by using the
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:doc:`set image <set>` command.
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The simplest way to output the results of the compute omega/chunk
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calculation to a file is to use the :doc:`fix ave/time <fix_ave_time>`
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command, for example:
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.. code-block:: LAMMPS
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compute cc1 all chunk/atom molecule
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compute myChunk all omega/chunk cc1
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fix 1 all ave/time 100 1 100 c_myChunk[*] file tmp.out mode vector
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Output info
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"""""""""""
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This compute calculates a global array where the number of rows is the
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number of chunks *Nchunk* as calculated by the specified :doc:`compute chunk/atom <compute_chunk_atom>` command. The number of columns is 3 for the three
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(*x*, *y*, *z*) components of the angular velocity for each chunk.
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These values can be accessed by any command that uses global array
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values from a compute as input. See the :doc:`Howto output <Howto_output>`
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page for an overview of LAMMPS output options.
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The array values are "intensive". The array values will be in
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velocity/distance :doc:`units <units>`.
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Restrictions
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""""""""""""
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none
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Related commands
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""""""""""""""""
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:doc:`variable omega() function <variable>`
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Default
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"""""""
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none
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