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<div class="section" id="compute-pair-local-command">
<span id="index-0"></span><h1>compute pair/local command</h1>
<div class="section" id="syntax">
<h2>Syntax</h2>
<div class="highlight-default"><div class="highlight"><pre><span></span><span class="n">compute</span> <span class="n">ID</span> <span class="n">group</span><span class="o">-</span><span class="n">ID</span> <span class="n">pair</span><span class="o">/</span><span class="n">local</span> <span class="n">value1</span> <span class="n">value2</span> <span class="o">...</span> <span class="n">keyword</span> <span class="n">args</span> <span class="o">...</span>
</pre></div>
</div>
<ul class="simple">
<li>ID, group-ID are documented in <a class="reference internal" href="compute.html"><span class="doc">compute</span></a> command</li>
<li>pair/local = style name of this compute command</li>
<li>one or more values may be appended</li>
<li>value = <em>dist</em> or <em>eng</em> or <em>force</em> or <em>fx</em> or <em>fy</em> or <em>fz</em> or <em>pN</em></li>
</ul>
<pre class="literal-block">
<em>dist</em> = pairwise distance
<em>eng</em> = pairwise energy
<em>force</em> = pairwise force
<em>fx</em>,<em>fy</em>,<em>fz</em> = components of pairwise force
<em>pN</em> = pair style specific quantities for allowed N values
</pre>
<ul class="simple">
<li>zero or more keyword/arg pairs may be appended</li>
<li>keyword = <em>cutoff</em></li>
</ul>
<pre class="literal-block">
<em>cutoff</em> arg = <em>type</em> or <em>radius</em>
</pre>
</div>
<div class="section" id="examples">
<h2>Examples</h2>
<div class="highlight-default"><div class="highlight"><pre><span></span><span class="n">compute</span> <span class="mi">1</span> <span class="nb">all</span> <span class="n">pair</span><span class="o">/</span><span class="n">local</span> <span class="n">eng</span>
<span class="n">compute</span> <span class="mi">1</span> <span class="nb">all</span> <span class="n">pair</span><span class="o">/</span><span class="n">local</span> <span class="n">dist</span> <span class="n">eng</span> <span class="n">force</span>
<span class="n">compute</span> <span class="mi">1</span> <span class="nb">all</span> <span class="n">pair</span><span class="o">/</span><span class="n">local</span> <span class="n">dist</span> <span class="n">eng</span> <span class="n">fx</span> <span class="n">fy</span> <span class="n">fz</span>
<span class="n">compute</span> <span class="mi">1</span> <span class="nb">all</span> <span class="n">pair</span><span class="o">/</span><span class="n">local</span> <span class="n">dist</span> <span class="n">fx</span> <span class="n">fy</span> <span class="n">fz</span> <span class="n">p1</span> <span class="n">p2</span> <span class="n">p3</span>
</pre></div>
</div>
</div>
<div class="section" id="description">
<h2>Description</h2>
<p>Define a computation that calculates properties of individual pairwise
interactions. The number of datums generated, aggregated across all
processors, equals the number of pairwise interactions in the system.</p>
<p>The local data stored by this command is generated by looping over the
pairwise neighbor list. Info about an individual pairwise interaction
will only be included if both atoms in the pair are in the specified
compute group, and if the current pairwise distance is less than the
force cutoff distance for that interaction, as defined by the
<a class="reference internal" href="pair_style.html"><span class="doc">pair_style</span></a> and <a class="reference internal" href="pair_coeff.html"><span class="doc">pair_coeff</span></a>
commands.</p>
<p>The value <em>dist</em> is the distance bewteen the pair of atoms.</p>
<p>The value <em>eng</em> is the interaction energy for the pair of atoms.</p>
<p>The value <em>force</em> is the force acting between the pair of atoms, which
is positive for a repulsive force and negative for an attractive
force. The values <em>fx</em>, <em>fy</em>, and <em>fz</em> are the xyz components of
<em>force</em> on atom I.</p>
<p>A pair style may define additional pairwise quantities which can be
accessed as <em>p1</em> to <em>pN</em>, where N is defined by the pair style. Most
pair styles do not define any additional quantities, so N = 0. An
example of ones that do are the <a class="reference internal" href="pair_gran.html"><span class="doc">granular pair styles</span></a>
which calculate the tangential force between two particles and return
its components and magnitude acting on atom I for N = 1,2,3,4. See
individual pair styles for detils.</p>
<p>The value <em>dist</em> will be in distance <a class="reference internal" href="units.html"><span class="doc">units</span></a>. The value
<em>eng</em> will be in energy <a class="reference internal" href="units.html"><span class="doc">units</span></a>. The values <em>force</em>, <em>fx</em>,
<em>fy</em>, and <em>fz</em> will be in force <a class="reference internal" href="units.html"><span class="doc">units</span></a>. The values <em>pN</em>
will be in whatever units the pair style defines.</p>
<p>The optional <em>cutoff</em> keyword determines how the force cutoff distance
for an interaction is determined. For the default setting of <em>type</em>,
the pairwise cutoff defined by the <a class="reference internal" href="pair_style.html"><span class="doc">pair_style</span></a>
command for the types of the two atoms is used. For the <em>radius</em>
setting, the sum of the radii of the two particles is used as a
cutoff. For example, this is appropriate for granular particles which
only interact when they are overlapping, as computed by <a class="reference external" href="pair_gran.txt">granular pair styles</a>.</p>
<p>Note that as atoms migrate from processor to processor, there will be
no consistent ordering of the entries within the local vector or array
from one timestep to the next. The only consistency that is
guaranteed is that the ordering on a particular timestep will be the
same for local vectors or arrays generated by other compute commands.
For example, pair output from the <a class="reference internal" href="compute_property_local.html"><span class="doc">compute property/local</span></a> command can be combined
with data from this command and output by the <a class="reference internal" href="dump.html"><span class="doc">dump local</span></a>
command in a consistent way.</p>
<p>Here is an example of how to do this:</p>
<div class="highlight-default"><div class="highlight"><pre><span></span><span class="n">compute</span> <span class="mi">1</span> <span class="nb">all</span> <span class="nb">property</span><span class="o">/</span><span class="n">local</span> <span class="n">patom1</span> <span class="n">patom2</span>
<span class="n">compute</span> <span class="mi">2</span> <span class="nb">all</span> <span class="n">pair</span><span class="o">/</span><span class="n">local</span> <span class="n">dist</span> <span class="n">eng</span> <span class="n">force</span>
<span class="n">dump</span> <span class="mi">1</span> <span class="nb">all</span> <span class="n">local</span> <span class="mi">1000</span> <span class="n">tmp</span><span class="o">.</span><span class="n">dump</span> <span class="n">index</span> <span class="n">c_1</span><span class="p">[</span><span class="mi">1</span><span class="p">]</span> <span class="n">c_1</span><span class="p">[</span><span class="mi">2</span><span class="p">]</span> <span class="n">c_2</span><span class="p">[</span><span class="mi">1</span><span class="p">]</span> <span class="n">c_2</span><span class="p">[</span><span class="mi">2</span><span class="p">]</span> <span class="n">c_2</span><span class="p">[</span><span class="mi">3</span><span class="p">]</span>
</pre></div>
</div>
<div class="admonition note">
<p class="first admonition-title">Note</p>
<p class="last">For pairs, if two atoms I,J are involved in 1-2, 1-3, 1-4
interactions within the molecular topology, their pairwise interaction
may be turned off, and thus they may not appear in the neighbor list,
and will not be part of the local data created by this command. More
specifically, this will be true of I,J pairs with a weighting factor
of 0.0; pairs with a non-zero weighting factor are included. The
weighting factors for 1-2, 1-3, and 1-4 pairwise interactions are set
by the <a class="reference internal" href="special_bonds.html"><span class="doc">special_bonds</span></a> command. An exception is if
long-range Coulombics are being computed via the
<a class="reference internal" href="kspace_style.html"><span class="doc">kspace_style</span></a> command, then atom pairs with
weighting factors of zero are still included in the neighbor list, so
that a portion of the long-range interaction contribution can be
computed in the pair style. Hence in that case, those atom pairs will
be part of the local data created by this command.</p>
</div>
<p><strong>Output info:</strong></p>
<p>This compute calculates a local vector or local array depending on the
number of keywords. The length of the vector or number of rows in the
array is the number of pairs. If a single keyword is specified, a
local vector is produced. If two or more keywords are specified, a
local array is produced where the number of columns = the number of
keywords. The vector or array can be accessed by any command that
uses local values from a compute as input. See <a class="reference internal" href="Section_howto.html#howto-15"><span class="std std-ref">this section</span></a> for an overview of LAMMPS output
options.</p>
<p>The output for <em>dist</em> will be in distance <a class="reference internal" href="units.html"><span class="doc">units</span></a>. The
output for <em>eng</em> will be in energy <a class="reference internal" href="units.html"><span class="doc">units</span></a>. The output for
<em>force</em>, <em>fx</em>, <em>fy</em>, and <em>fz</em> will be in force <a class="reference internal" href="units.html"><span class="doc">units</span></a>.
The outpur for <em>pN</em> will be in whatever units the pair style defines.</p>
</div>
<div class="section" id="restrictions">
<h2>Restrictions</h2>
<blockquote>
<div>none</div></blockquote>
</div>
<div class="section" id="related-commands">
<h2>Related commands</h2>
<p><a class="reference internal" href="dump.html"><span class="doc">dump local</span></a>, <a class="reference internal" href="compute_property_local.html"><span class="doc">compute property/local</span></a></p>
</div>
<div class="section" id="default">
<h2>Default</h2>
<p>The keyword default is cutoff = type.</p>
</div>
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