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