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<div class="section" id="fix-spring-rg-command">
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<span id="index-0"></span><h1>fix spring/rg 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">fix</span> <span class="n">ID</span> <span class="n">group</span><span class="o">-</span><span class="n">ID</span> <span class="n">spring</span><span class="o">/</span><span class="n">rg</span> <span class="n">K</span> <span class="n">RG0</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="fix.html"><span class="doc">fix</span></a> command</li>
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<li>spring/rg = style name of this fix command</li>
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<li>K = harmonic force constant (force/distance units)</li>
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<li>RG0 = target radius of gyration to constrain to (distance units)</li>
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</ul>
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<div class="highlight-default"><div class="highlight"><pre><span></span><span class="k">if</span> <span class="n">RG0</span> <span class="o">=</span> <span class="n">NULL</span><span class="p">,</span> <span class="n">use</span> <span class="n">the</span> <span class="n">current</span> <span class="n">RG</span> <span class="k">as</span> <span class="n">the</span> <span class="n">target</span> <span class="n">value</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="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">fix</span> <span class="mi">1</span> <span class="n">protein</span> <span class="n">spring</span><span class="o">/</span><span class="n">rg</span> <span class="mf">5.0</span> <span class="mf">10.0</span>
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<span class="n">fix</span> <span class="mi">2</span> <span class="n">micelle</span> <span class="n">spring</span><span class="o">/</span><span class="n">rg</span> <span class="mf">5.0</span> <span class="n">NULL</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>Apply a harmonic restraining force to atoms in the group to affect
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their central moment about the center of mass (radius of gyration).
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This fix is useful to encourage a protein or polymer to fold/unfold
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and also when sampling along the radius of gyration as a reaction
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coordinate (i.e. for protein folding).</p>
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<p>The radius of gyration is defined as RG in the first formula. The
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energy of the constraint and associated force on each atom is given by
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the second and third formulas, when the group is at a different RG
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than the target value RG0.</p>
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<img alt="_images/fix_spring_rg.jpg" class="align-center" src="_images/fix_spring_rg.jpg" />
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<p>The (xi - center-of-mass) term is computed taking into account
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periodic boundary conditions, m_i is the mass of the atom, and M is
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the mass of the entire group. Note that K is thus a force constant
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for the aggregate force on the group of atoms, not a per-atom force.</p>
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<p>If RG0 is specified as NULL, then the RG of the group is computed at
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the time the fix is specified, and that value is used as the target.</p>
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</div>
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<div class="section" id="restart-fix-modify-output-run-start-stop-minimize-info">
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<h2>Restart, fix_modify, output, run start/stop, minimize info</h2>
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<p>No information about this fix is written to <a class="reference internal" href="restart.html"><span class="doc">binary restart files</span></a>. None of the <a class="reference internal" href="fix_modify.html"><span class="doc">fix_modify</span></a> options
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are relevant to this fix. No global or per-atom quantities are stored
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by this fix for access by various <a class="reference internal" href="Section_howto.html#howto-15"><span class="std std-ref">output commands</span></a>. No parameter of this fix can
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be used with the <em>start/stop</em> keywords of the <a class="reference internal" href="run.html"><span class="doc">run</span></a> command.
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This fix is not invoked during <a class="reference internal" href="minimize.html"><span class="doc">energy minimization</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="fix_spring.html"><span class="doc">fix spring</span></a>, <a class="reference internal" href="fix_spring_self.html"><span class="doc">fix spring/self</span></a>
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<a class="reference internal" href="fix_drag.html"><span class="doc">fix drag</span></a>, <a class="reference internal" href="fix_smd.html"><span class="doc">fix smd</span></a></p>
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<p><strong>Default:</strong> none</p>
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© Copyright 2013 Sandia Corporation.
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