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<title>compute property/chunk command &mdash; LAMMPS 15 Dec 2015 documentation</title>
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<div class="section" id="compute-property-chunk-command">
<span id="index-0"></span><h1>compute property/chunk command<a class="headerlink" href="#compute-property-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 property/chunk chunkID input1 input2 ...
</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>property/chunk = style name of this compute command</li>
<li>input = one or more attributes</li>
</ul>
<pre class="literal-block">
attributes = count, id, coord1, coord2, coord3
count = # of atoms in chunk
id = original chunk IDs before compression by <a class="reference internal" href="compute_chunk_atom.html"><em>compute chunk/atom</em></a>
coord123 = coordinates for spatial bins calculated by <a class="reference internal" href="compute_chunk_atom.html"><em>compute chunk/atom</em></a>
</pre>
</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 all property/chunk count
compute 1 all property/chunk ID coord1
</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 stores the specified attributes of 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 and stores the specified attributes of chunks
as global data so they can be accessed by other <a class="reference internal" href="Section_howto.html#howto-15"><span>output commands</span></a> and used in conjunction with
other commands that generate per-chunk data, such as <a class="reference internal" href="compute_com_chunk.html"><em>compute com/chunk</em></a> or <a class="reference internal" href="compute_msd_chunk.html"><em>compute msd/chunk</em></a>.</p>
<p>Note that only atoms in the specified group contribute to the
calculation of the <em>count</em> attribute. 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>
<p>The <em>count</em> attribute is the number of atoms in the chunk.</p>
<p>The <em>id</em> attribute stores the original chunk ID for each chunk. It
can only be used if the <em>compress</em> keyword was set to <em>yes</em> for the
<a class="reference internal" href="compute_chunk_atom.html"><em>compute chunk/atom</em></a> command referenced by
chunkID. This means that the original chunk IDs (e.g. molecule IDs)
will have been compressed to remove chunk IDs with no atoms assigned
to them. Thus a compresed chunk ID of 3 may correspond to an original
chunk ID (molecule ID in this case) of 415. The <em>id</em> attribute will
then be 415 for the 3rd chunk.</p>
<p>The <em>coordN</em> attributes can only be used if a <em>binning</em> style was used
in the <a class="reference internal" href="compute_chunk_atom.html"><em>compute chunk/atom</em></a> command referenced
by chunkID. For <em>bin/1d</em>, <em>bin/2d</em>, and <em>bin/3d</em> styles the attribute
is the center point of the bin in the corresponding dimension. Style
<em>bin/1d</em> only defines a <em>coord1</em> attribute. Style <em>bin/2d</em> adds a
<em>coord2</em> attribute. Style <em>bin/3d</em> adds a <em>coord3</em> attribute.</p>
<p>Note that if the value of the <em>units</em> keyword used in the <a class="reference internal" href="compute_chunk_atom.html"><em>compute chunk/atom command</em></a> is <em>box</em> or <em>lattice</em>, the
<em>coordN</em> attributes will be in distance <a class="reference internal" href="units.html"><em>units</em></a>. If the
value of the <em>units</em> keyword is <em>reduced</em>, the <em>coordN</em> attributes
will be in unitless reduced units (0-1).</p>
<p>The simplest way to output the results of the compute property/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 myChunk1 all property/chunk cc1
compute myChunk2 all com/chunk cc1
fix 1 all ave/time 100 1 100 c_myChunk1 c_myChunk2 file tmp.out mode vector
</pre></div>
</div>
<p><strong>Output info:</strong></p>
<p>This compute calculates a global vector or global array depending on
the number of input values. The length of the vector or number of
rows in the array is the number of chunks.</p>
<p>This compute calculates a global vector or 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. If a
single input is specified, a global vector is produced. If two or
more inputs are specified, a global array is produced where the number
of columns = the number of inputs. The vector or array can be
accessed by any command that uses global values from a compute as
input. See <a class="reference internal" href="Section_howto.html#howto-15"><span>this section</span></a> for an overview
of LAMMPS output options.</p>
<p>The vector or array values are &#8220;intensive&#8221;. The values will be
unitless or in the units discussed above.</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="fix_ave_chunk.html"><em>fix ave/chunk</em></a></p>
<p><strong>Default:</strong> none</p>
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