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<div class="section" id="fix-evaporate-command">
<span id="index-0"></span><h1>fix evaporate 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">evaporate</span> <span class="n">N</span> <span class="n">M</span> <span class="n">region</span><span class="o">-</span><span class="n">ID</span> <span class="n">seed</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>evaporate = style name of this fix command</li>
<li>N = delete atoms every this many timesteps</li>
<li>M = number of atoms to delete each time</li>
<li>region-ID = ID of region within which to perform deletions</li>
<li>seed = random number seed to use for choosing atoms to delete</li>
<li>zero or more keyword/value pairs may be appended</li>
</ul>
<pre class="literal-block">
keyword = <em>molecule</em>
<em>molecule</em> value = <em>no</em> or <em>yes</em>
</pre>
</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="mi">1</span> <span class="n">solvent</span> <span class="n">evaporate</span> <span class="mi">1000</span> <span class="mi">10</span> <span class="n">surface</span> <span class="mi">49892</span>
<span class="n">fix</span> <span class="mi">1</span> <span class="n">solvent</span> <span class="n">evaporate</span> <span class="mi">1000</span> <span class="mi">10</span> <span class="n">surface</span> <span class="mi">38277</span> <span class="n">molecule</span> <span class="n">yes</span>
</pre></div>
</div>
</div>
<div class="section" id="description">
<h2>Description</h2>
<p>Remove M atoms from the simulation every N steps. This can be used,
for example, to model evaporation of solvent particles or moleclues
(i.e. drying) of a system. Every N steps, the number of atoms in the
fix group and within the specifed region are counted. M of these are
chosen at random and deleted. If there are less than M eligible
particles, then all of them are deleted.</p>
<p>If the setting for the <em>molecule</em> keyword is <em>no</em>, then only single
atoms are deleted. In this case, you should insure you do not delete
only a portion of a molecule (only some of its atoms), or LAMMPS will
soon generate an error when it tries to find those atoms. LAMMPS will
warn you if any of the atoms eligible for deletion have a non-zero
molecule ID, but does not check for this at the time of deletion.</p>
<p>If the setting for the <em>molecule</em> keyword is <em>yes</em>, then when an atom
is chosen for deletion, the entire molecule it is part of is deleted.
The count of deleted atoms is incremented by the number of atoms in
the molecule, which may make it exceed <em>M</em>. If the molecule ID of the
chosen atom is 0, then it is assumed to not be part of a molecule, and
just the single atom is deleted.</p>
<p>As an example, if you wish to delete 10 water molecules every <em>N</em>
steps, you should set <em>M</em> to 30. If only the water&#8217;s oxygen atoms
were in the fix group, then two hydrogen atoms would be deleted when
an oxygen atom is selected for deletion, whether the hydrogens are
inside the evaporation region or not.</p>
<p>Note that neighbor lists are re-built on timesteps that atoms are
removed. Thus you should not remove atoms too frequently or you will
incur overhead due to the cost of building neighbor lists.</p>
<div class="admonition note">
<p class="first admonition-title">Note</p>
<p class="last">If you are monitoring the temperature of a system where the atom
count is changing due to evaporation, you typically should use the
<a class="reference internal" href="compute_modify.html"><span class="doc">compute_modify dynamic yes</span></a> command for the
temperature compute you are using.</p>
</div>
</div>
<div class="section" id="restart-fix-modify-output-run-start-stop-minimize-info">
<h2>Restart, fix_modify, output, run start/stop, minimize info</h2>
<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
are relevant to this fix.</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 the
cummulative number of deleted atoms. The scalar value calculated by
this fix is &#8220;intensive&#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. This fix is not invoked during <a class="reference internal" href="minimize.html"><span class="doc">energy minimization</span></a>.</p>
</div>
<div class="section" id="restrictions">
<h2>Restrictions</h2>
<p>This fix is part of the MISC package. It is only enabled if LAMMPS
was built with that package. See the <a class="reference internal" href="Section_start.html#start-3"><span class="std std-ref">Making LAMMPS</span></a> section for more info.</p>
</div>
<div class="section" id="related-commands">
<h2>Related commands</h2>
<p><a class="reference internal" href="fix_deposit.html"><span class="doc">fix deposit</span></a></p>
</div>
<div class="section" id="default">
<h2>Default</h2>
<p>The option defaults are molecule = no.</p>
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