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<li class="toctree-l2"><a class="reference internal" href="Howto.html#analysis-howto">8.3. Analysis howto</a></li>
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<li class="toctree-l2"><a class="reference internal" href="Howto.html#force-fields-howto">8.4. Force fields howto</a></li>
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<li class="toctree-l1"><a class="reference internal" href="Modify.html">3. Modifying & extending LAMMPS</a></li>
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<li class="breadcrumb-item active"><span class="section-number">8.1.4. </span>Multi-replica simulations</li>
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<p><span class="math notranslate nohighlight">\(\renewcommand{\AA}{\text{Å}}\)</span></p>
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<section id="multi-replica-simulations">
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<h1><span class="section-number">8.1.4. </span>Multi-replica simulations<a class="headerlink" href="#multi-replica-simulations" title="Link to this heading"></a></h1>
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<p>Several commands in LAMMPS run multi-replica simulations, meaning
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that multiple instances (replicas) of your simulation are run
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simultaneously, with small amounts of data exchanged between replicas
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periodically.</p>
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<p>These are the relevant commands:</p>
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<ul class="simple">
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<li><p><a class="reference internal" href="hyper.html"><span class="doc">hyper</span></a> for bond boost hyperdynamics (HD)</p></li>
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<li><p><a class="reference internal" href="neb.html"><span class="doc">neb</span></a> for nudged elastic band calculations (NEB)</p></li>
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<li><p><a class="reference internal" href="neb_spin.html"><span class="doc">neb_spin</span></a> for magnetic nudged elastic band calculations</p></li>
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<li><p><a class="reference internal" href="prd.html"><span class="doc">prd</span></a> for parallel replica dynamics (PRD)</p></li>
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<li><p><a class="reference internal" href="tad.html"><span class="doc">tad</span></a> for temperature accelerated dynamics (TAD)</p></li>
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<li><p><a class="reference internal" href="temper.html"><span class="doc">temper</span></a> for parallel tempering with fixed volume</p></li>
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<li><p><a class="reference internal" href="temper_npt.html"><span class="doc">temper/npt</span></a> for parallel tempering extended for NPT</p></li>
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<li><p><a class="reference internal" href="temper_grem.html"><span class="doc">temper/grem</span></a> for parallel tempering with generalized replica exchange (gREM)</p></li>
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<li><p><a class="reference internal" href="fix_pimd.html"><span class="doc">fix pimd</span></a> for path-integral molecular dynamics (PIMD)</p></li>
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</ul>
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<p>NEB is a method for finding transition states and barrier potential energies.
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HD, PRD, and TAD are methods for performing accelerated dynamics to find and
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perform infrequent events. Parallel tempering or replica exchange runs
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different replicas at a series of temperature to facilitate rare-event
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sampling. PIMD runs different replicas whose individual particles in different
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replicas are coupled together by springs to model a system of ring-polymers which
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can represent the quantum nature of atom cores.</p>
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<p>These commands can only be used if LAMMPS was built with the REPLICA
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package. See the <a class="reference internal" href="Build_package.html"><span class="doc">Build package</span></a> page for more
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info.</p>
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<p>In all these cases, you must run with one or more processors per
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replica. The processors assigned to each replica are determined at
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run-time by using the <a class="reference internal" href="Run_options.html"><span class="doc">-partition command-line switch</span></a> to launch LAMMPS on multiple partitions, which in this
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context are the same as replicas. E.g. these commands:</p>
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<div class="highlight-bash notranslate"><div class="highlight"><pre><span></span>mpirun<span class="w"> </span>-np<span class="w"> </span><span class="m">16</span><span class="w"> </span>lmp_linux<span class="w"> </span>-partition<span class="w"> </span>8x2<span class="w"> </span>-in<span class="w"> </span><span class="k">in</span>.temper
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mpirun<span class="w"> </span>-np<span class="w"> </span><span class="m">8</span><span class="w"> </span>lmp_linux<span class="w"> </span>-partition<span class="w"> </span>8x1<span class="w"> </span>-in<span class="w"> </span><span class="k">in</span>.neb
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</pre></div>
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</div>
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<p>would each run 8 replicas, on either 16 or 8 processors. Note the use
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of the <a class="reference internal" href="Run_options.html"><span class="doc">-in command-line switch</span></a> to specify the input
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script which is required when running in multi-replica mode.</p>
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<p>Also note that with MPI installed on a machine (e.g. your desktop), you
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can run on more (virtual) processors than you have physical processors.
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Thus, the above commands could be run on a single-processor (or
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few-processor) desktop so that you can run a multi-replica simulation on
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more replicas than you have physical processors. This is useful for
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testing and debugging, since with most modern processors and MPI
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libraries, the efficiency of a calculation can severely diminish when
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oversubscribing processors.</p>
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