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<li><a class="reference internal" href="#">fix ext_alignment command</a><ul>
<li><a class="reference internal" href="#syntax">Syntax</a></li>
<li><a class="reference internal" href="#examples">Examples</a></li>
<li><a class="reference internal" href="#description">Description</a></li>
<li><a class="reference internal" href="#restart-output-run-start-stop">Restart, output, run start/stop</a></li>
<li><a class="reference internal" href="#restrictions">Restrictions</a></li>
<li><a class="reference internal" href="#related-commands">Related commands</a></li>
<li><a class="reference internal" href="#default">Default</a></li>
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  <section id="fix-ext-alignment-command">
<h1>fix ext_alignment command<a class="headerlink" href="#fix-ext-alignment-command" title="Permalink to this heading"></a></h1>
<section id="syntax">
<h2>Syntax<a class="headerlink" href="#syntax" title="Permalink to this heading"></a></h2>
<div class="highlight-default notranslate"><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">ext_alignment</span> <span class="n">particle_axis</span> <span class="n">fdx</span> <span class="n">fdy</span> <span class="n">fdz</span> <span class="n">force_constant</span>
</pre></div>
</div>
<ul>
<li><p>ID, group-ID are documented in <a class="reference external" href="https://docs.lammps.org/fix.html">fix</a> command</p></li>
<li><p>ext_alignment = style name of this fix command</p></li>
<li><p>particle_axis = <em>angle_f</em> or <em>angle_v</em> or <em>angle_u</em></p></li>
<li><p>fdx,fdy,fdz = external field component values (distance units)</p></li>
<li><p>force_constant = alignment force constant magnitude (force units)</p>
<blockquote>
<div></div></blockquote>
</li>
</ul>
</section>
<section id="examples">
<h2>Examples<a class="headerlink" href="#examples" title="Permalink to this heading"></a></h2>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">fix</span> <span class="n">torque</span> <span class="nb">all</span> <span class="n">ext_alignment</span> <span class="n">angle_u</span> <span class="mi">1</span> <span class="mi">1</span> <span class="mi">0</span> <span class="mf">90.0</span>
<span class="n">fix</span> <span class="n">torque</span> <span class="n">crystals</span> <span class="n">ext_alignment</span> <span class="n">angle_v</span> <span class="mi">0</span> <span class="mi">1</span> <span class="mi">0</span> <span class="mf">150.0</span>
<span class="n">fix</span> <span class="n">field</span> <span class="n">polymer</span> <span class="n">ext_alignment</span> <span class="n">angle_u</span> <span class="mi">0</span> <span class="mi">0</span> <span class="mi">1</span> <span class="mf">1000.0</span>
</pre></div>
</div>
</section>
<section id="description">
<h2>Description<a class="headerlink" href="#description" title="Permalink to this heading"></a></h2>
<p>Add a torque to a specified axis of a particular group of finite-size particles with respect to a fixed direction.
The applied torque is as follows:</p>
<div class="math notranslate nohighlight">
\[
{\bf\tau} =
\left\{
\begin{array}{
  &#64;{}% no padding
  l&#64;{\quad}% some padding
  r&#64;{}% no padding
  &gt;{{}}r&#64;{}% no padding
  &gt;{{}}l&#64;{}% no padding
}
  k({\hat{u}_i}{\times}{\hat{\psi}}),&amp;         &amp; {\rm if} &amp; {\hat{u}_i}{\cdot}{\hat{\psi}}&gt;0 \\
  -k({\hat{u}_i}{\times}{\hat{\psi}}),&amp;        &amp; {\rm if} &amp; {\hat{u}_i}{\cdot}{\hat{\psi}}&lt;0
\end{array}
\right.
\]</div><p>were <span class="math notranslate nohighlight">\({k}\)</span> is the force constant that determines how aggressive is the external alignment field, <span class="math notranslate nohighlight">\({\hat{u}_i}\)</span> is the axis of the finite-size particle to be aligned with respect to the direction of the external field, and <span class="math notranslate nohighlight">\({\hat{\psi}}\)</span> is the unit directional vector. The direction of <span class="math notranslate nohighlight">\({\hat{\psi}}\)</span> can be set in any direction of
a fixed 3D space.
This torque allows a parallel alignment of the particles with
respect to the field, regardless of whether their main axis (<span class="math notranslate nohighlight">\({\hat{u}_i}\)</span>) is pointing in the opposite direction as <span class="math notranslate nohighlight">\({\hat{\psi}}\)</span>.</p>
<p>This command can be used to induce an artificial alignment to finite-size particles in a simulation, such as for liquid crystal simulations of uniaxial or biaxial particles.</p>
<p><em>particle_axis</em> assumes that the orientation of each site in the specified group-ID  is given by an orthonormal basis ( <span class="math notranslate nohighlight">\({\hat{f}}\)</span> , <span class="math notranslate nohighlight">\({\hat{v}}\)</span> , <span class="math notranslate nohighlight">\({\hat{u}}\)</span>), and therefore one must specify the direction to be oriented.</p>
<p>External field component values can take any desired values since they are normalized in the code, but it is suggested to use numbers from 0 to 1.</p>
<hr class="docutils" />
<p>Note that this fix can only act on finite-size particles. Therefore, there must be an Ellipsoids section in the LAMMPS input to obtain the orientation of the particles.
See <a class="reference external" href="https://docs.lammps.org/atom_style.html">atom_style command</a>  ellipsoid.</p>
</section>
<section id="restart-output-run-start-stop">
<h2>Restart, output, run start/stop<a class="headerlink" href="#restart-output-run-start-stop" title="Permalink to this heading"></a></h2>
<p>No information about this fix is written to <a class="reference external" href="https://docs.lammps.org/restart.html">binary restart files</a>.</p>
<p>No parameter of this fix can be used with the start/stop keywords of the run command.</p>
</section>
<section id="restrictions">
<h2>Restrictions<a class="headerlink" href="#restrictions" title="Permalink to this heading"></a></h2>
<p>This fix does not support equal-style or atom-style <a class="reference external" href="https://docs.lammps.org/variable.html">variable</a>.</p>
<blockquote>
<div></div></blockquote>
<p>Particles acted on by the potential can be finite-size aspherical or spherical particles.</p>
<p>This fix is only enabled if LAMMPS was compiled with fix_ext_alignment.cpp and fix_ext_alignment.h.</p>
<div class="admonition note">
<p class="admonition-title">Note</p>
<p>This fix is currently supported in the latest LAMMPS stable release 23 Jun 2022.
See the <a class="reference external" href="https://www.lammps.org/download.html">LAMMPS</a> website for more info.</p>
</div>
</section>
<section id="related-commands">
<h2>Related commands<a class="headerlink" href="#related-commands" title="Permalink to this heading"></a></h2>
<p><a class="reference external" href="https://docs.lammps.org/fix_addforce.html">fix addforce</a>, <a class="reference external" href="https://docs.lammps.org/fix_efield.html">fix efield</a></p>
<blockquote>
<div></div></blockquote>
</section>
<section id="default">
<h2>Default<a class="headerlink" href="#default" title="Permalink to this heading"></a></h2>
<p>none</p>
<div class="toctree-wrapper compound">
</div>
</section>
</section>


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