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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_start.html">2. Getting Started</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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<li class="toctree-l1"><a class="reference internal" href="Section_errors.html">12. Errors</a></li>
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<div class="section" id="compute-group-group-command">
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<span id="index-0"></span><h1>compute group/group command<a class="headerlink" href="#compute-group-group-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 group/group group2-ID keyword value ...
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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>group/group = style name of this compute command</li>
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<li>group2-ID = group ID of second (or same) group</li>
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<li>zero or more keyword/value pairs may be appended</li>
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<li>keyword = <em>pair</em> or <em>kspace</em> or <em>boundary</em></li>
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</ul>
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<pre class="literal-block">
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<em>pair</em> value = <em>yes</em> or <em>no</em>
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<em>kspace</em> value = <em>yes</em> or <em>no</em>
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<em>boundary</em> value = <em>yes</em> or <em>no</em>
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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<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 lower group/group upper
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compute 1 lower group/group upper kspace yes
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compute mine fluid group/group wall
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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 total energy and force
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interaction between two groups of atoms: the compute group and the
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specified group2. The two groups can be the same.</p>
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<p>If the <em>pair</em> keyword is set to <em>yes</em>, which is the default, then the
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the interaction energy will include a pair component which is defined
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as the pairwise energy between all pairs of atoms where one atom in
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the pair is in the first group and the other is in the second group.
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Likewise, the interaction force calculated by this compute will
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include the force on the compute group atoms due to pairwise
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interactions with atoms in the specified group2.</p>
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<p>If the <em>kspace</em> keyword is set to <em>yes</em>, which is not the default, and
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if a <a class="reference internal" href="kspace_style.html"><em>kspace_style</em></a> is defined, then the interaction
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energy will include a Kspace component which is the long-range
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Coulombic energy between all the atoms in the first group and all the
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atoms in the 2nd group. Likewise, the interaction force calculated by
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this compute will include the force on the compute group atoms due to
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long-range Coulombic interactions with atoms in the specified group2.</p>
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<p>Normally the long-range Coulombic energy converges only when the net
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charge of the unit cell is zero. However, one can assume the net
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charge of the system is neutralized by a uniform background plasma,
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and a correction to the system energy can be applied to reduce
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artifacts. For more information see <a class="reference internal" href="#bogusz"><span>(Bogusz)</span></a>. If the
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<em>boundary</em> keyword is set to <em>yes</em>, which is the default, and <em>kspace</em>
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contributions are included, then this energy correction term will be
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added to the total group-group energy. This correction term does not
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affect the force calculation and will be zero if one or both of the
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groups are charge neutral. This energy correction term is the same as
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that included in the regular Ewald and PPPM routines.</p>
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<p>This compute does not calculate any bond or angle or dihedral or
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improper interactions between atoms in the two groups.</p>
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<hr class="docutils" />
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<p>The pairwise contributions to the group-group interactions are
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calculated by looping over a neighbor list. The Kspace contribution
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to the group-group interactions require essentially the same amount of
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work (FFTs, Ewald summation) as computing long-range forces for the
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entire system. Thus it can be costly to invoke this compute too
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frequently.</p>
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<p>If you desire a breakdown of the interactions into a pairwise and
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Kspace component, simply invoke the compute twice with the appropriate
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yes/no settings for the <em>pair</em> and <em>kspace</em> keywords. This is no more
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costly than using a single compute with both keywords set to <em>yes</em>.
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The individual contributions can be summed in a
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<a class="reference internal" href="variable.html"><em>variable</em></a> if desired.</p>
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<p>This <a class="reference external" href="PDF/kspace.pdf">document</a> describes how the long-range
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group-group calculations are performed.</p>
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<hr class="docutils" />
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<p><strong>Output info:</strong></p>
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<p>This compute calculates a global scalar (the energy) and a global
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vector of length 3 (force), which can be accessed by indices 1-3.
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These values can be used by any command that uses global scalar or
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vector 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
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options.</p>
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<p>Both the scalar and vector values calculated by this compute are
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“extensive”. The scalar value will be in energy <a class="reference internal" href="units.html"><em>units</em></a>.
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The vector values will be in force <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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<p>Not all pair styles can be evaluated in a pairwise mode as required by
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this compute. For example, 3-body and other many-body potentials,
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such as <a class="reference internal" href="pair_tersoff.html"><em>Tersoff</em></a> and
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<a class="reference internal" href="pair_sw.html"><em>Stillinger-Weber</em></a> cannot be used. <a class="reference internal" href="pair_eam.html"><em>EAM</em></a>
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potentials only include the pair potential portion of the EAM
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interaction when used by this compute, not the embedding term.</p>
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<p>Not all Kspace styles support calculation of group/group interactions.
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The <em>ewald</em> and <em>pppm</em> styles do.</p>
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<p><strong>Related commands:</strong> none</p>
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</div>
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<div class="section" id="default">
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<h2>Default<a class="headerlink" href="#default" title="Permalink to this headline">¶</a></h2>
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<p>The option defaults are pair = yes, kspace = no, and boundary = yes.</p>
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<hr class="docutils" />
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<p id="bogusz">Bogusz et al, J Chem Phys, 108, 7070 (1998)</p>
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