89 lines
2.7 KiB
ReStructuredText
89 lines
2.7 KiB
ReStructuredText
.. index:: compute vcm/chunk
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compute vcm/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 vcm/chunk chunkID
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* ID, group-ID are documented in :doc:`compute <compute>` command
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* vcm/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 vcm/chunk molchunk
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Description
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"""""""""""
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Define a computation that calculates the center-of-mass velocity for
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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 as
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chunkID. For example, a single chunk could be the atoms in a molecule or atoms
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in a spatial bin. See the :doc:`compute chunk/atom <compute_chunk_atom>` and
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:doc:`Howto chunk <Howto_chunk>` doc pages for details of how chunks can be
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defined and examples of how they can be used to measure properties of a system.
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This compute calculates the :math:`(x,y,z)` components of the center-of-mass
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velocity for each chunk. This is done by summing mass\*velocity for
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each atom in the chunk and dividing the sum by the total mass of the
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chunk.
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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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The simplest way to output the results of the compute vcm/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 vcm/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
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:doc:`compute chunk/atom <compute_chunk_atom>` command. The number of
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columns is 3 for the :math:`(x,y,z)` center-of-mass velocity coordinates of
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each chunk. 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 :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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none
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Default
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"""""""
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none
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