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<li class="toctree-l1"><a class="reference internal" href="Section_intro.html">1. Introduction</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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<div class="section" id="compute-pair-local-command">
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<span id="index-0"></span><h1>compute pair/local command</h1>
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<div class="section" id="syntax">
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<h2>Syntax</h2>
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<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">input1</span> <span class="n">input2</span> <span class="o">...</span>
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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"><span class="doc">compute</span></a> command</li>
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<li>pair/local = style name of this compute command</li>
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<li>zero or more keywords may be appended</li>
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<li>keyword = <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>
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</ul>
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<pre class="literal-block">
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<em>dist</em> = pairwise distance
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<em>eng</em> = pairwise energy
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<em>force</em> = pairwise force
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<em>fx</em>,<em>fy</em>,<em>fz</em> = components of pairwise force
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<em>pN</em> = pair style specific quantities for allowed N values
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</pre>
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</div>
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<div class="section" id="examples">
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<h2>Examples</h2>
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<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>
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<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>
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<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>
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<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>
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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</h2>
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<p>Define a computation that calculates properties of individual pairwise
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interactions. The number of datums generated, aggregated across all
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processors, equals the number of pairwise interactions in the system.</p>
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<p>The local data stored by this command is generated by looping over the
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pairwise neighbor list. Info about an individual pairwise interaction
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will only be included if both atoms in the pair are in the specified
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compute group, and if the current pairwise distance is less than the
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force cutoff distance for that interaction, as defined by the
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<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>
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commands.</p>
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<p>The output <em>dist</em> is the distance bewteen the pair of atoms.</p>
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<p>The output <em>eng</em> is the interaction energy for the pair of atoms.</p>
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<p>The output <em>force</em> is the force acting between the pair of atoms,
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which is positive for a repulsive force and negative for an attractive
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force. The outputs <em>fx</em>, <em>fy</em>, and <em>fz</em> are the xyz components of
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<em>force</em> on atom I.</p>
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<p>A pair style may define additional pairwise quantities which can be
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accessed as <em>p1</em> to <em>pN</em>, where N is defined by the pair style. Most
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pair styles do not define any additional quantities, so N = 0. An
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example of ones that do are the <a class="reference internal" href="pair_gran.html"><span class="doc">granular pair styles</span></a>
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which calculate the tangential force between two particles and return
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its components and magnitude acting on atom I for N = 1,2,3,4. See
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individual pair styles for detils.</p>
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<p>The output <em>dist</em> will be in distance <a class="reference internal" href="units.html"><span class="doc">units</span></a>. The output
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<em>eng</em> will be in energy <a class="reference internal" href="units.html"><span class="doc">units</span></a>. The outputs <em>force</em>,
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<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 output
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<em>pN</em> will be in whatever units the pair style defines.</p>
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<p>Note that as atoms migrate from processor to processor, there will be
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no consistent ordering of the entries within the local vector or array
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from one timestep to the next. The only consistency that is
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guaranteed is that the ordering on a particular timestep will be the
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same for local vectors or arrays generated by other compute commands.
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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
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with data from this command and output by the <a class="reference internal" href="dump.html"><span class="doc">dump local</span></a>
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command in a consistent way.</p>
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<div class="admonition note">
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<p class="first admonition-title">Note</p>
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<p class="last">For pairs, if two atoms I,J are involved in 1-2, 1-3, 1-4
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interactions within the molecular topology, their pairwise interaction
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may be turned off, and thus they may not appear in the neighbor list,
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and will not be part of the local data created by this command. More
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specifically, this will be true of I,J pairs with a weighting factor
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of 0.0; pairs with a non-zero weighting factor are included. The
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weighting factors for 1-2, 1-3, and 1-4 pairwise interactions are set
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by the <a class="reference internal" href="special_bonds.html"><span class="doc">special_bonds</span></a> command. An exception is if
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long-range Coulombics are being computed via the
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<a class="reference internal" href="kspace_style.html"><span class="doc">kspace_style</span></a> command, then atom pairs with
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weighting factors of zero are still included in the neighbor list, so
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that a portion of the long-range interaction contribution can be
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computed in the pair style. Hence in that case, those atom pairs will
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be part of the local data created by this command.</p>
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</div>
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<p><strong>Output info:</strong></p>
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<p>This compute calculates a local vector or local array depending on the
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number of keywords. The length of the vector or number of rows in the
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array is the number of pairs. If a single keyword is specified, a
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local vector is produced. If two or more keywords are specified, a
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local array is produced where the number of columns = the number of
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keywords. The vector or array can be accessed by any command that
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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
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options.</p>
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<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
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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
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<em>force</em> will be in force <a class="reference internal" href="units.html"><span class="doc">units</span></a>.</p>
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</div>
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<div class="section" id="restrictions">
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<h2>Restrictions</h2>
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<blockquote>
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<div>none</div></blockquote>
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</div>
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<div class="section" id="related-commands">
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<h2>Related commands</h2>
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<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>
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<p><strong>Default:</strong> none</p>
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