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<div class="section" id="compute-com-chunk-command">
<span id="index-0"></span><h1>compute com/chunk command<a class="headerlink" href="#compute-com-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 com/chunk chunkID
</pre></div>
</div>
<ul class="simple">
<li>ID, group-ID are documented in <a class="reference internal" href="compute.html"><em>compute</em></a> command</li>
<li>com/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>
</ul>
</div>
<div class="section" id="examples">
<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 com/chunk molchunk
</pre></div>
</div>
</div>
<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 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
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 <a class="reference internal" href="compute_chunk_atom.html"><em>compute chunk/atom</em></a> doc page and &#8220;<a class="reference internal" href="Section_howto.html#howto-23"><span>Section_howto 23</span></a> for details of how chunks can be
defined and examples of how they can be used to measure properties of
a system.</p>
<p>This compute calculates the x,y,z coordinates of the center-of-mass
for each chunk, which includes all effects due to atoms passing thru
periodic boundaries.</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
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
&#8220;all&#8221; group for this command if you simply want to include atoms with
non-zero chunk IDs.</p>
<div class="admonition warning">
<p class="first admonition-title">Warning</p>
<p class="last">The coordinates of an atom contribute to the chunk&#8217;s
center-of-mass in &#8220;unwrapped&#8221; form, by using the image flags
associated with each atom. See the <a class="reference internal" href="dump.html"><em>dump custom</em></a> command
for a discussion of &#8220;unwrapped&#8221; coordinates. See the Atoms section of
the <a class="reference internal" href="read_data.html"><em>read_data</em></a> 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 <a class="reference internal" href="set.html"><em>set image</em></a> command.</p>
</div>
<p>The simplest way to output the results of the compute com/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
compute myChunk all com/chunk cc1
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 =
3 for the x,y,z center-of-mass coordinates of each chunk. These
values can be accessed by any command that uses global array 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 &#8220;intensive&#8221;. The array values will be in
distance <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>
</div>
<div class="section" id="related-commands">
<h2>Related commands<a class="headerlink" href="#related-commands" title="Permalink to this headline">¶</a></h2>
<p><a class="reference internal" href="compute_com.html"><em>compute com</em></a></p>
<p><strong>Default:</strong> none</p>
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