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<div class="section" id="fix-spring-self-command">
<span id="index-0"></span><h1>fix spring/self command</h1>
<div class="section" id="syntax">
<h2>Syntax</h2>
<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="bp">self</span> <span class="n">K</span> <span class="nb">dir</span>
</pre></div>
</div>
<ul class="simple">
<li>ID, group-ID are documented in <a class="reference internal" href="fix.html"><span class="doc">fix</span></a> command</li>
<li>spring/self = style name of this fix command</li>
<li>K = spring constant (force/distance units)</li>
<li>dir = xyz, xy, xz, yz, x, y, or z (optional, default: xyz)</li>
</ul>
</div>
<div class="section" id="examples">
<h2>Examples</h2>
<div class="highlight-default"><div class="highlight"><pre><span></span><span class="n">fix</span> <span class="n">tether</span> <span class="n">boundary</span><span class="o">-</span><span class="n">atoms</span> <span class="n">spring</span><span class="o">/</span><span class="bp">self</span> <span class="mf">10.0</span>
<span class="n">fix</span> <span class="n">zrest</span> <span class="n">move</span> <span class="n">spring</span><span class="o">/</span><span class="bp">self</span> <span class="mf">10.0</span> <span class="n">z</span>
</pre></div>
</div>
</div>
<div class="section" id="description">
<h2>Description</h2>
<p>Apply a spring force independently to each atom in the group to tether
it to its initial position. The initial position for each atom is its
location at the time the fix command was issued. At each timestep,
the magnitude of the force on each atom is -Kr, where r is the
displacement of the atom from its current position to its initial
position. The distance r correctly takes into account any crossings
of periodic boundary by the atom since it was in its intitial
position.</p>
<p>With the (optional) dir flag, one can select in which direction the
spring force is applied. By default, the restraint is applied in all
directions, but it can be limited to the xy-, xz-, yz-plane and the
x-, y-, or z-direction, thus restraining the atoms to a line or a
plane, respectively.</p>
<p><strong>Restart, fix_modify, output, run start/stop, minimize info:</strong></p>
<p>This fix writes the original coordinates of tethered atoms to <a class="reference internal" href="restart.html"><span class="doc">binary restart files</span></a>, so that the spring effect will be the
same in a restarted simulation. See the
<a class="reference internal" href="read_restart.html"><span class="doc">read_restart</span></a> command for info on how to re-specify
a fix in an input script that reads a restart file, so that the
operation of the fix continues in an uninterrupted fashion.</p>
<p>The <a class="reference internal" href="fix_modify.html"><span class="doc">fix_modify</span></a> <em>energy</em> option is supported by this
fix to add the energy stored in the per-atom springs to the system&#8217;s
potential energy as part of <a class="reference internal" href="thermo_style.html"><span class="doc">thermodynamic output</span></a>.</p>
<p>The <a class="reference internal" href="fix_modify.html"><span class="doc">fix_modify</span></a> <em>respa</em> option is supported by
this fix. This allows to set at which level of the <a class="reference internal" href="run_style.html"><span class="doc">r-RESPA</span></a>
integrator the fix is adding its forces. Default is the outermost level.</p>
<p>This fix computes a global scalar which can be accessed by various
<a class="reference internal" href="Section_howto.html#howto-15"><span class="std std-ref">output commands</span></a>. The scalar is an
energy which is the sum of the spring energy for each atom, where the
per-atom energy is 0.5 * K * r^2. The scalar value calculated by this
fix is &#8220;extensive&#8221;.</p>
<p>No parameter of this fix can 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.</p>
<p>The forces due to this fix are imposed during an energy minimization,
invoked by the <a class="reference internal" href="minimize.html"><span class="doc">minimize</span></a> command.</p>
<div class="admonition note">
<p class="first admonition-title">Note</p>
<p class="last">If you want the per-atom spring energy to be included in the
total potential energy of the system (the quantity being minimized),
you MUST enable the <a class="reference internal" href="fix_modify.html"><span class="doc">fix_modify</span></a> <em>energy</em> option for
this fix.</p>
</div>
</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="fix_drag.html"><span class="doc">fix drag</span></a>, <a class="reference internal" href="fix_spring.html"><span class="doc">fix spring</span></a>,
<a class="reference internal" href="fix_smd.html"><span class="doc">fix smd</span></a>, <a class="reference internal" href="fix_spring_rg.html"><span class="doc">fix spring/rg</span></a></p>
<p><strong>Default:</strong> none</p>
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