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<div class="section" id="fix-nve-limit-command">
<span id="index-0"></span><h1>fix nve/limit 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">nve</span><span class="o">/</span><span class="n">limit</span> <span class="n">xmax</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>nve = style name of this fix command</li>
<li>xmax = maximum distance an atom can move in one timestep (distance units)</li>
</ul>
</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="nb">all</span> <span class="n">nve</span><span class="o">/</span><span class="n">limit</span> <span class="mf">0.1</span>
</pre></div>
</div>
</div>
<div class="section" id="description">
<h2>Description</h2>
<p>Perform constant NVE updates of position and velocity for atoms in the
group each timestep. A limit is imposed on the maximum distance an
atom can move in one timestep. This is useful when starting a
simulation with a configuration containing highly overlapped atoms.
Normally this would generate huge forces which would blow atoms out of
the simulation box, causing LAMMPS to stop with an error.</p>
<p>Using this fix can overcome that problem. Forces on atoms must still
be computable (which typically means 2 atoms must have a separation
distance &gt; 0.0). But large velocities generated by large forces are
reset to a value that corresponds to a displacement of length <em>xmax</em>
in a single timestep. <em>Xmax</em> is specified in distance units; see the
<span class="xref doc">units</span> command for details. The value of <em>xmax</em> should be
consistent with the neighbor skin distance and the frequency of
neighbor list re-building, so that pairwise interactions are not
missed on successive timesteps as atoms move. See the
<a class="reference internal" href="neighbor.html"><span class="doc">neighbor</span></a> and <a class="reference internal" href="neigh_modify.html"><span class="doc">neigh_modify</span></a> commands
for details.</p>
<p>Note that if a velocity reset occurs the integrator will not conserve
energy. On steps where no velocity resets occur, this integrator is
exactly like the <a class="reference internal" href="fix_nve.html"><span class="doc">fix nve</span></a> command. Since forces are
unaltered, pressures computed by thermodynamic output will still be
very large for overlapped configurations.</p>
<div class="admonition note">
<p class="first admonition-title">Note</p>
<p class="last">You should not use <a class="reference internal" href="fix_shake.html"><span class="doc">fix shake</span></a> in conjunction
with this fix. That is because fix shake applies contraint forces
based on the predicted postitions of atoms after the next timestep.
It has no way of knowing the timestep may change due to this fix,
which will cause the constraint forces to be invalid. A better
strategy is to turn off fix shake when performing initial dynamics
that need this fix, then turn fix shake on when doing normal dynamics
with a fixed-size timestep.</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
count of how many updates of atom&#8217;s velocity/position were limited by
the maximum distance criterion. This should be roughly the number of
atoms so affected, except that updates occur at both the beginning and
end of a timestep in a velocity Verlet timestepping algorithm. This
is a cumulative quantity for the current run, but is re-initialized to
zero each time a run is performed. The scalar value calculated by
this fix is &#8220;extensive&#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="fix_nve.html"><span class="doc">fix nve</span></a>, <a class="reference internal" href="fix_nve_noforce.html"><span class="doc">fix nve/noforce</span></a>,
<a class="reference internal" href="pair_soft.html"><span class="doc">pair_style soft</span></a></p>
<p><strong>Default:</strong> none</p>
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