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<div class="section" id="fix-dt-reset-command">
<span id="index-0"></span><h1>fix dt/reset 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">dt</span><span class="o">/</span><span class="n">reset</span> <span class="n">N</span> <span class="n">Tmin</span> <span class="n">Tmax</span> <span class="n">Xmax</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>dt/reset = style name of this fix command</li>
<li>N = recompute dt every N timesteps</li>
<li>Tmin = minimum dt allowed which can be NULL (time units)</li>
<li>Tmax = maximum dt allowed which can be NULL (time units)</li>
<li>Xmax = maximum distance for an atom to move in one timestep (distance units)</li>
<li>zero or more keyword/value pairs may be appended</li>
<li>keyword = <em>units</em></li>
</ul>
<pre class="literal-block">
<em>units</em> value = <em>lattice</em> or <em>box</em>
lattice = Xmax is defined in lattice units
box = Xmax is defined in simulation box units
</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">5</span> <span class="nb">all</span> <span class="n">dt</span><span class="o">/</span><span class="n">reset</span> <span class="mi">10</span> <span class="mf">1.0e-5</span> <span class="mf">0.01</span> <span class="mf">0.1</span>
<span class="n">fix</span> <span class="mi">5</span> <span class="nb">all</span> <span class="n">dt</span><span class="o">/</span><span class="n">reset</span> <span class="mi">10</span> <span class="mf">0.01</span> <span class="mf">2.0</span> <span class="mf">0.2</span> <span class="n">units</span> <span class="n">box</span>
</pre></div>
</div>
</div>
<div class="section" id="description">
<h2>Description</h2>
<p>Reset the timestep size every N steps during a run, so that no atom
moves further than Xmax, based on current atom velocities and forces.
This can be useful when starting from a configuration with overlapping
atoms, where forces will be large. Or it can be useful when running
an impact simulation where one or more high-energy atoms collide with
a solid, causing a damage cascade.</p>
<p>This fix overrides the timestep size setting made by the
<a class="reference internal" href="timestep.html"><span class="doc">timestep</span></a> command. The new timestep size <em>dt</em> is
computed in the following manner.</p>
<p>For each atom, the timestep is computed that would cause it to
displace <em>Xmax</em> on the next integration step, as a function of its
current velocity and force. Since performing this calculation exactly
would require the solution to a quartic equation, a cheaper estimate
is generated. The estimate is conservative in that the atom&#8217;s
displacement is guaranteed not to exceed <em>Xmax</em>, though it may be
smaller.</p>
<p>Given this putative timestep for each atom, the minimum timestep value
across all atoms is computed. Then the <em>Tmin</em> and <em>Tmax</em> bounds are
applied, if specified. If one (or both) is specified as NULL, it is
not applied.</p>
<p>When the <a class="reference internal" href="run_style.html"><span class="doc">run style</span></a> is <em>respa</em>, this fix resets the
outer loop (largest) timestep, which is the same timestep that the
<a class="reference internal" href="timestep.html"><span class="doc">timestep</span></a> command sets.</p>
<p>Note that the cumulative simulation time (in time units), which
accounts for changes in the timestep size as a simulation proceeds,
can be accessed by the <a class="reference internal" href="thermo_style.html"><span class="doc">thermo_style time</span></a> keyword.</p>
</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 stores
the last timestep on which the timestep was reset to a new value.</p>
<p>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>
<blockquote>
<div>none</div></blockquote>
</div>
<div class="section" id="related-commands">
<h2>Related commands</h2>
<p><a class="reference internal" href="timestep.html"><span class="doc">timestep</span></a></p>
</div>
<div class="section" id="default">
<h2>Default</h2>
<p>The option defaults is units = lattice.</p>
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