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<title>compute vcm/chunk command — LAMMPS 15 May 2015 version documentation</title>
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<li>compute vcm/chunk command</li>
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<div class="section" id="compute-vcm-chunk-command">
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<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>
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<div class="section" id="syntax">
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<h2>Syntax<a class="headerlink" href="#syntax" title="Permalink to this headline">¶</a></h2>
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<div class="highlight-python"><div class="highlight"><pre>compute ID group-ID vcm/chunk chunkID
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</pre></div>
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</div>
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<ul class="simple">
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<li>ID, group-ID are documented in <a class="reference internal" href="compute.html"><em>compute</em></a> command</li>
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<li>vcm/chunk = style name of this compute command</li>
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<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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</ul>
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</div>
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<div class="section" id="examples">
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<h2>Examples<a class="headerlink" href="#examples" title="Permalink to this headline">¶</a></h2>
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<div class="highlight-python"><div class="highlight"><pre>compute 1 fluid vcm/chunk molchunk
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</pre></div>
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</div>
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</div>
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<div class="section" id="description">
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<h2>Description<a class="headerlink" href="#description" title="Permalink to this headline">¶</a></h2>
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<p>Define a computation that calculates the center-of-mass velocity for
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multiple chunks of atoms.</p>
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<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
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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 <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>
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<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
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each atom in the chunk and dividing the sum by the total mass of the
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chunk.</p>
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<p>Note that only atoms in the specified group contribute to the
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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
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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.</p>
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<p>The simplest way to output the results of the compute vcm/chunk
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calculation to a file is to use the <a class="reference internal" href="fix_ave_time.html"><em>fix ave/time</em></a>
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command, for example:</p>
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<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
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</pre></div>
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</div>
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<p><strong>Output info:</strong></p>
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<p>This compute calculates a global array where the number of rows = the
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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.
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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 <a class="reference internal" href="Section_howto.html#howto-15"><span>Section_howto 15</span></a> for an overview of LAMMPS output
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options.</p>
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<p>The array values are “intensive”. The array values will be in
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velocity <a class="reference internal" href="units.html"><em>units</em></a>.</p>
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</div>
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<div class="section" id="restrictions">
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<h2>Restrictions<a class="headerlink" href="#restrictions" title="Permalink to this headline">¶</a></h2>
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<blockquote>
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<div>none</div></blockquote>
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<p><strong>Related commands:</strong> none</p>
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<p><strong>Default:</strong> none</p>
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