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<div class="section" id="fix-langevin-eff-command">
<span id="index-0"></span><h1>fix langevin/eff 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">langevin</span><span class="o">/</span><span class="n">eff</span> <span class="n">Tstart</span> <span class="n">Tstop</span> <span class="n">damp</span> <span class="n">seed</span> <span class="n">keyword</span> <span class="n">values</span> <span class="o">...</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>langevin/eff = style name of this fix command</li>
<li>Tstart,Tstop = desired temperature at start/end of run (temperature units)</li>
<li>damp = damping parameter (time units)</li>
<li>seed = random number seed to use for white noise (positive integer)</li>
<li>zero or more keyword/value pairs may be appended</li>
</ul>
<pre class="literal-block">
keyword = <em>scale</em> or <em>tally</em> or <em>zero</em>
<em>scale</em> values = type ratio
type = atom type (1-N)
ratio = factor by which to scale the damping coefficient
<em>tally</em> values = <em>no</em> or <em>yes</em>
<em>no</em> = do not tally the energy added/subtracted to atoms
<em>yes</em> = do tally the energy added/subtracted to atoms
</pre>
<pre class="literal-block">
<em>zero</em> value = <em>no</em> or <em>yes</em>
<em>no</em> = do not set total random force to zero
<em>yes</em> = set total random force to zero
</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">3</span> <span class="n">boundary</span> <span class="n">langevin</span><span class="o">/</span><span class="n">eff</span> <span class="mf">1.0</span> <span class="mf">1.0</span> <span class="mf">10.0</span> <span class="mi">699483</span>
<span class="n">fix</span> <span class="mi">1</span> <span class="nb">all</span> <span class="n">langevin</span><span class="o">/</span><span class="n">eff</span> <span class="mf">1.0</span> <span class="mf">1.1</span> <span class="mf">10.0</span> <span class="mi">48279</span> <span class="n">scale</span> <span class="mi">3</span> <span class="mf">1.5</span>
</pre></div>
</div>
</div>
<div class="section" id="description">
<h2>Description</h2>
<p>Apply a Langevin thermostat as described in <a class="reference internal" href="#schneider"><span class="std std-ref">(Schneider)</span></a>
to a group of nuclei and electrons in the <a class="reference internal" href="pair_eff.html"><span class="doc">electron force field</span></a> model. Used with <a class="reference internal" href="fix_nve_eff.html"><span class="doc">fix nve/eff</span></a>,
this command performs Brownian dynamics (BD), since the total force on
each atom will have the form:</p>
<div class="highlight-default"><div class="highlight"><pre><span></span><span class="n">F</span> <span class="o">=</span> <span class="n">Fc</span> <span class="o">+</span> <span class="n">Ff</span> <span class="o">+</span> <span class="n">Fr</span>
<span class="n">Ff</span> <span class="o">=</span> <span class="o">-</span> <span class="p">(</span><span class="n">m</span> <span class="o">/</span> <span class="n">damp</span><span class="p">)</span> <span class="n">v</span>
<span class="n">Fr</span> <span class="ow">is</span> <span class="n">proportional</span> <span class="n">to</span> <span class="n">sqrt</span><span class="p">(</span><span class="n">Kb</span> <span class="n">T</span> <span class="n">m</span> <span class="o">/</span> <span class="p">(</span><span class="n">dt</span> <span class="n">damp</span><span class="p">))</span>
</pre></div>
</div>
<p>Fc is the conservative force computed via the usual inter-particle
interactions (<a class="reference internal" href="pair_style.html"><span class="doc">pair_style</span></a>).</p>
<p>The Ff and Fr terms are added by this fix on a per-particle basis.</p>
<p>The operation of this fix is exactly like that described by the <a class="reference internal" href="fix_langevin.html"><span class="doc">fix langevin</span></a> command, except that the thermostatting
is also applied to the radial electron velocity for electron
particles.</p>
<p><strong>Restart, fix_modify, output, run start/stop, minimize info:</strong></p>
<p>No information about this fix is written to <a class="reference internal" href="restart.html"><span class="doc">binary restart files</span></a>. Because the state of the random number generator
is not saved in restart files, this means you cannot do &#8220;exact&#8221;
restarts with this fix, where the simulation continues on the same as
if no restart had taken place. However, in a statistical sense, a
restarted simulation should produce the same behavior.</p>
<p>The <a class="reference internal" href="fix_modify.html"><span class="doc">fix_modify</span></a> <em>temp</em> option is supported by this
fix. You can use it to assign a temperature <a class="reference internal" href="compute.html"><span class="doc">compute</span></a>
you have defined to this fix which will be used in its thermostatting
procedure, as described above. For consistency, the group used by
this fix and by the compute should be the same.</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 change induced by Langevin thermostatting 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>. Note that use of this option requires
setting the <em>tally</em> keyword to <em>yes</em>.</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 energy change due to this fix. The scalar value
calculated by this fix is &#8220;extensive&#8221;. Note that calculation of this
quantity requires setting the <em>tally</em> keyword to <em>yes</em>.</p>
<p>This fix can ramp its target temperature over multiple runs, using the
<em>start</em> and <em>stop</em> keywords of the <a class="reference internal" href="run.html"><span class="doc">run</span></a> command. See the
<a class="reference internal" href="run.html"><span class="doc">run</span></a> command for details of how to do this.</p>
<p>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>
<blockquote>
<div>none</div></blockquote>
<p>This fix is part of the USER-EFF 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_langevin.html"><span class="doc">fix langevin</span></a></p>
</div>
<div class="section" id="default">
<h2>Default</h2>
<p>The option defaults are scale = 1.0 for all types and tally = no.</p>
<hr class="docutils" />
<p id="dunweg"><strong>(Dunweg)</strong> Dunweg and Paul, Int J of Modern Physics C, 2, 817-27 (1991).</p>
<p id="schneider"><strong>(Schneider)</strong> Schneider and Stoll, Phys Rev B, 17, 1302 (1978).</p>
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