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