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<p><span class="math notranslate nohighlight">\(\renewcommand{\AA}{\text{Å}}\)</span></p>
<section id="restart-a-simulation">
<h1><span class="section-number">8.1.1. </span>Restart a simulation<a class="headerlink" href="#restart-a-simulation" title="Link to this heading"></a></h1>
<p>There are 3 ways to continue a long LAMMPS simulation. Multiple
<a class="reference internal" href="run.html"><span class="doc">run</span></a> commands can be used in the same input script. Each
run will continue from where the previous run left off. Or binary
restart files can be saved to disk using the <a class="reference internal" href="restart.html"><span class="doc">restart</span></a>
command. At a later time, these binary files can be read via a
<a class="reference internal" href="read_restart.html"><span class="doc">read_restart</span></a> command in a new script. Or they can
be converted to text data files using the <a class="reference internal" href="Run_options.html"><span class="doc">-r command-line switch</span></a> and read by a <a class="reference internal" href="read_data.html"><span class="doc">read_data</span></a> command in a
new script.</p>
<p>Here we give examples of 2 scripts that read either a binary restart
file or a converted data file and then issue a new run command to
continue where the previous run left off. They illustrate what
settings must be made in the new script. Details are discussed in the
documentation for the <a class="reference internal" href="read_restart.html"><span class="doc">read_restart</span></a> and
<a class="reference internal" href="read_data.html"><span class="doc">read_data</span></a> commands.</p>
<p>Look at the <em>in.chain</em> input script provided in the <em>bench</em> directory
of the LAMMPS distribution to see the original script that these 2
scripts are based on. If that script had the line</p>
<div class="highlight-LAMMPS notranslate"><div class="highlight"><pre><span></span><span class="k">restart</span><span class="w"> </span><span class="m">50</span><span class="w"> </span><span class="n">tmp.restart</span>
</pre></div>
</div>
<p>added to it, it would produce two binary restart files (<code class="docutils literal notranslate"><span class="pre">tmp.restart.50</span></code>
and <code class="docutils literal notranslate"><span class="pre">tmp.restart.100</span></code>) as it ran.</p>
<p>This script could be used to read the first restart file and re-run the
last 50 timesteps:</p>
<div class="highlight-LAMMPS notranslate"><div class="highlight"><pre><span></span><span class="k">read_restart</span><span class="w"> </span><span class="n">tmp.restart.50</span>
<span class="k">neighbor</span><span class="w"> </span><span class="m">0.4</span><span class="w"> </span><span class="n">bin</span>
<span class="k">neigh_modify</span><span class="w"> </span><span class="n">every</span><span class="w"> </span><span class="m">1</span><span class="w"> </span><span class="n">delay</span><span class="w"> </span><span class="m">1</span>
<span class="k">fix </span><span class="nv nv-Identifier">1</span><span class="w"> </span><span class="nv nv-Identifier">all</span><span class="w"> </span><span class="n">nve</span>
<span class="k">fix </span><span class="nv nv-Identifier">2</span><span class="w"> </span><span class="nv nv-Identifier">all</span><span class="w"> </span><span class="n">langevin</span><span class="w"> </span><span class="m">1.0</span><span class="w"> </span><span class="m">1.0</span><span class="w"> </span><span class="m">10.0</span><span class="w"> </span><span class="m">904297</span>
<span class="k">timestep</span><span class="w"> </span><span class="m">0.012</span>
<span class="k">run</span><span class="w"> </span><span class="m">50</span>
</pre></div>
</div>
<p>Note that the following commands do not need to be repeated because
their settings are included in the restart file: <code class="code highlight LAMMPS docutils literal highlight-LAMMPS"><span class="k">units</span></code>, <code class="code highlight LAMMPS docutils literal highlight-LAMMPS"><span class="k">atom_style</span></code>,
<code class="code highlight LAMMPS docutils literal highlight-LAMMPS"><span class="k">special_bonds</span></code>, <code class="code highlight LAMMPS docutils literal highlight-LAMMPS"><span class="k">pair_style</span></code>, <code class="code highlight LAMMPS docutils literal highlight-LAMMPS"><span class="k">bond_style</span></code>. However, these commands do
need to be used, since their settings are not in the restart file:
<code class="code highlight LAMMPS docutils literal highlight-LAMMPS"><span class="k">neighbor</span></code>, <code class="code highlight LAMMPS docutils literal highlight-LAMMPS"><span class="k">fix</span></code>, <code class="code highlight LAMMPS docutils literal highlight-LAMMPS"><span class="k">timestep</span></code>.</p>
<p>If you actually use this script to perform a restarted run, you will
notice that the thermodynamic data match at step 50 (if you also put a
<code class="code highlight LAMMPS docutils literal highlight-LAMMPS"><span class="k">thermo</span><span class="w"> </span><span class="m">50</span></code> command in the original script), but do not match at step
100. This is because the <a class="reference internal" href="fix_langevin.html"><span class="doc">fix langevin</span></a> command
uses random numbers in a way that does not allow for perfect restarts.</p>
<p>As an alternate approach, the restart file could be converted to a data
file as follows:</p>
<div class="highlight-bash notranslate"><div class="highlight"><pre><span></span>lmp_g++<span class="w"> </span>-r<span class="w"> </span>tmp.restart.50<span class="w"> </span>tmp.restart.data
</pre></div>
</div>
<p>Then, this script could be used to re-run the last 50 steps:</p>
<div class="highlight-LAMMPS notranslate"><div class="highlight"><pre><span></span><span class="k">units</span><span class="w"> </span><span class="n">lj</span>
<span class="k">atom_style</span><span class="w"> </span><span class="n">bond</span>
<span class="k">pair_style</span><span class="w"> </span><span class="n">lj</span><span class="o">/</span><span class="n">cut</span><span class="w"> </span><span class="m">1.12</span>
<span class="k">pair_modify</span><span class="w"> </span><span class="n">shift</span><span class="w"> </span><span class="n">yes</span>
<span class="k">bond_style</span><span class="w"> </span><span class="n">fene</span>
<span class="k">special_bonds</span><span class="w"> </span><span class="m">0.0</span><span class="w"> </span><span class="m">1.0</span><span class="w"> </span><span class="m">1.0</span>
<span class="k">read_data</span><span class="w"> </span><span class="n">tmp.restart.data</span>
<span class="k">neighbor</span><span class="w"> </span><span class="m">0.4</span><span class="w"> </span><span class="n">bin</span>
<span class="k">neigh_modify</span><span class="w"> </span><span class="n">every</span><span class="w"> </span><span class="m">1</span><span class="w"> </span><span class="n">delay</span><span class="w"> </span><span class="m">1</span>
<span class="k">fix </span><span class="nv nv-Identifier">1</span><span class="w"> </span><span class="nv nv-Identifier">all</span><span class="w"> </span><span class="n">nve</span>
<span class="k">fix </span><span class="nv nv-Identifier">2</span><span class="w"> </span><span class="nv nv-Identifier">all</span><span class="w"> </span><span class="n">langevin</span><span class="w"> </span><span class="m">1.0</span><span class="w"> </span><span class="m">1.0</span><span class="w"> </span><span class="m">10.0</span><span class="w"> </span><span class="m">904297</span>
<span class="k">timestep</span><span class="w"> </span><span class="m">0.012</span>
<span class="k">reset_timestep</span><span class="w"> </span><span class="m">50</span>
<span class="k">run</span><span class="w"> </span><span class="m">50</span>
</pre></div>
</div>
<p>Note that nearly all the settings specified in the original <code class="docutils literal notranslate"><span class="pre">in.chain</span></code>
script must be repeated, except the <code class="code highlight LAMMPS docutils literal highlight-LAMMPS"><span class="k">pair_coeff</span></code> and <code class="code highlight LAMMPS docutils literal highlight-LAMMPS"><span class="k">bond_coeff</span></code>
commands, since the new data file lists the force field coefficients.
Also, the <a class="reference internal" href="reset_timestep.html"><span class="doc">reset_timestep</span></a> command is used to tell
LAMMPS the current timestep. This value is stored in restart files, but
not in data files.</p>
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