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<div class="section" id="fix-spring-self-command">
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<span id="index-0"></span><h1>fix spring/self 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="bp">self</span> <span class="n">K</span> <span class="nb">dir</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/self = style name of this fix command</li>
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<li>K = spring constant (force/distance units)</li>
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<li>dir = xyz, xy, xz, yz, x, y, or z (optional, default: xyz)</li>
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</ul>
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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="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>
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<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>
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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 spring force independently to each atom in the group to tether
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it to its initial position. The initial position for each atom is its
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location at the time the fix command was issued. At each timestep,
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the magnitude of the force on each atom is -Kr, where r is the
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displacement of the atom from its current position to its initial
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position. The distance r correctly takes into account any crossings
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of periodic boundary by the atom since it was in its intitial
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position.</p>
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<p>With the (optional) dir flag, one can select in which direction the
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spring force is applied. By default, the restraint is applied in all
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directions, but it can be limited to the xy-, xz-, yz-plane and the
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x-, y-, or z-direction, thus restraining the atoms to a line or a
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plane, respectively.</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>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
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same in a restarted simulation. See the
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<a class="reference internal" href="read_restart.html"><span class="doc">read_restart</span></a> command for info on how to re-specify
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a fix in an input script that reads a restart file, so that the
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operation of the fix continues in an uninterrupted fashion.</p>
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<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
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fix to add the energy stored in the per-atom springs to the system’s
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potential energy as part of <a class="reference internal" href="thermo_style.html"><span class="doc">thermodynamic output</span></a>.</p>
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<p>This fix computes a global scalar which can be accessed by various
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<a class="reference internal" href="Section_howto.html#howto-15"><span class="std std-ref">output commands</span></a>. The scalar is an
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energy which is the sum of the spring energy for each atom, where the
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per-atom energy is 0.5 * K * r^2. The scalar value calculated by this
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fix is “extensive”.</p>
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<p>No parameter of this fix can be used with the <em>start/stop</em> keywords of
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the <a class="reference internal" href="run.html"><span class="doc">run</span></a> command.</p>
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<p>The forces due to this fix are imposed during an energy minimization,
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invoked by the <a class="reference internal" href="minimize.html"><span class="doc">minimize</span></a> command.</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">If you want the per-atom spring energy to be included in the
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total potential energy of the system (the quantity being minimized),
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you MUST enable the <a class="reference internal" href="fix_modify.html"><span class="doc">fix_modify</span></a> <em>energy</em> option for
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this fix.</p>
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</div>
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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_drag.html"><span class="doc">fix drag</span></a>, <a class="reference internal" href="fix_spring.html"><span class="doc">fix spring</span></a>,
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<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>
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<p><strong>Default:</strong> none</p>
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