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