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< title > compute vcm/chunk command — LAMMPS documentation< / title >
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< li class = "toctree-l1" > < a class = "reference internal" href = "Section_intro.html" > 1. Introduction< / a > < / li >
< li class = "toctree-l1" > < a class = "reference internal" href = "Section_start.html" > 2. Getting Started< / a > < / li >
< li class = "toctree-l1" > < a class = "reference internal" href = "Section_commands.html" > 3. Commands< / a > < / li >
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< li class = "toctree-l1" > < a class = "reference internal" href = "Section_accelerate.html" > 5. Accelerating LAMMPS performance< / a > < / li >
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< li class = "toctree-l1" > < a class = "reference internal" href = "Section_perf.html" > 8. Performance & scalability< / a > < / li >
< li class = "toctree-l1" > < a class = "reference internal" href = "Section_tools.html" > 9. Additional tools< / a > < / li >
< li class = "toctree-l1" > < a class = "reference internal" href = "Section_modify.html" > 10. Modifying & extending LAMMPS< / a > < / li >
< li class = "toctree-l1" > < a class = "reference internal" href = "Section_python.html" > 11. Python interface to LAMMPS< / a > < / li >
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< li > compute vcm/chunk command< / li >
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< a href = "http://lammps.sandia.gov" > Website< / a >
< a href = "Section_commands.html#comm" > Commands< / a >
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< div class = "section" id = "compute-vcm-chunk-command" >
< span id = "index-0" > < / span > < h1 > compute vcm/chunk command< a class = "headerlink" href = "#compute-vcm-chunk-command" title = "Permalink to this headline" > ¶< / a > < / h1 >
< div class = "section" id = "syntax" >
< h2 > Syntax< a class = "headerlink" href = "#syntax" title = "Permalink to this headline" > ¶< / a > < / h2 >
< div class = "highlight-python" > < div class = "highlight" > < pre > compute ID group-ID vcm/chunk chunkID
< / pre > < / div >
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< ul class = "simple" >
< li > ID, group-ID are documented in < a class = "reference internal" href = "compute.html" > < em > compute< / em > < / a > command< / li >
< li > vcm/chunk = style name of this compute command< / li >
< li > chunkID = ID of < a class = "reference internal" href = "compute_chunk_atom.html" > < em > compute chunk/atom< / em > < / a > command< / li >
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< h2 > Examples< a class = "headerlink" href = "#examples" title = "Permalink to this headline" > ¶< / a > < / h2 >
< div class = "highlight-python" > < div class = "highlight" > < pre > compute 1 fluid vcm/chunk molchunk
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< div class = "section" id = "description" >
< h2 > Description< a class = "headerlink" href = "#description" title = "Permalink to this headline" > ¶< / a > < / h2 >
< p > Define a computation that calculates the center-of-mass velocity for
multiple chunks of atoms.< / p >
< p > In LAMMPS, chunks are collections of atoms defined by a < a class = "reference internal" href = "compute_chunk_atom.html" > < em > compute chunk/atom< / em > < / a > command, which assigns each atom
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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
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molecule or atoms in a spatial bin. See the < a class = "reference internal" href = "compute_chunk_atom.html" > < em > compute chunk/atom< / em > < / a > doc page and “ < a class = "reference internal" href = "Section_howto.html#howto-23" > < span > Section_howto 23< / span > < / a > for details of how chunks can be
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defined and examples of how they can be used to measure properties of
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a system.< / p >
< p > This compute calculates the 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
each atom in the chunk and dividing the sum by the total mass of the
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chunk.< / p >
< p > Note that only atoms in the specified group contribute to the
calculation. The < a class = "reference internal" href = "compute_chunk_atom.html" > < em > compute chunk/atom< / em > < / a > command
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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
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“ all” group for this command if you simply want to include atoms with
non-zero chunk IDs.< / p >
< p > The simplest way to output the results of the compute vcm/chunk
calculation to a file is to use the < a class = "reference internal" href = "fix_ave_time.html" > < em > fix ave/time< / em > < / a >
command, for example:< / p >
< div class = "highlight-python" > < div class = "highlight" > < pre > 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
< / pre > < / div >
< / div >
< p > < strong > Output info:< / strong > < / p >
< p > This compute calculates a global array where the number of rows = the
number of chunks < em > Nchunk< / em > as calculated by the specified < a class = "reference internal" href = "compute_chunk_atom.html" > < em > compute chunk/atom< / em > < / a > command. The number of columns =
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3 for the x,y,z center-of-mass velocity coordinates of 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 < a class = "reference internal" href = "Section_howto.html#howto-15" > < span > Section_howto 15< / span > < / a > for an overview of LAMMPS output
options.< / p >
< p > The array values are “ intensive” . The array values will be in
velocity < a class = "reference internal" href = "units.html" > < em > units< / em > < / a > .< / p >
< / div >
< div class = "section" id = "restrictions" >
< h2 > Restrictions< a class = "headerlink" href = "#restrictions" title = "Permalink to this headline" > ¶< / a > < / h2 >
< blockquote >
< div > none< / div > < / blockquote >
< p > < strong > Related commands:< / strong > none< / p >
< p > < strong > Default:< / strong > none< / p >
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© Copyright 2013 Sandia Corporation.
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