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<li>restart command</li>
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<div class="section" id="restart-command">
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<span id="index-0"></span><h1>restart 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">restart</span> <span class="mi">0</span>
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<span class="n">restart</span> <span class="n">N</span> <span class="n">root</span> <span class="n">keyword</span> <span class="n">value</span> <span class="o">...</span>
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<span class="n">restart</span> <span class="n">N</span> <span class="n">file1</span> <span class="n">file2</span> <span class="n">keyword</span> <span class="n">value</span> <span class="o">...</span>
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</pre></div>
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</div>
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<ul class="simple">
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<li>N = write a restart file every this many timesteps</li>
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<li>N can be a variable (see below)</li>
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<li>root = filename to which timestep # is appended</li>
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<li>file1,file2 = two full filenames, toggle between them when writing file</li>
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<li>zero or more keyword/value pairs may be appended</li>
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<li>keyword = <em>fileper</em> or <em>nfile</em></li>
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</ul>
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<pre class="literal-block">
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<em>fileper</em> arg = Np
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Np = write one file for every this many processors
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<em>nfile</em> arg = Nf
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Nf = write this many files, one from each of Nf processors
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</pre>
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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">restart</span> <span class="mi">0</span>
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<span class="n">restart</span> <span class="mi">1000</span> <span class="n">poly</span><span class="o">.</span><span class="n">restart</span>
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<span class="n">restart</span> <span class="mi">1000</span> <span class="n">poly</span><span class="o">.</span><span class="n">restart</span><span class="o">.</span><span class="n">mpiio</span>
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<span class="n">restart</span> <span class="mi">1000</span> <span class="n">restart</span><span class="o">.*.</span><span class="n">equil</span>
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<span class="n">restart</span> <span class="mi">10000</span> <span class="n">poly</span><span class="o">.%.</span><span class="mi">1</span> <span class="n">poly</span><span class="o">.%.</span><span class="mi">2</span> <span class="n">nfile</span> <span class="mi">10</span>
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<span class="n">restart</span> <span class="n">v_mystep</span> <span class="n">poly</span><span class="o">.</span><span class="n">restart</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>Write out a binary restart file with the current state of the
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simulation every so many timesteps, in either or both of two modes, as
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a run proceeds. A value of 0 means do not write out any restart
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files. The two modes are as follows. If one filename is specified, a
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series of filenames will be created which include the timestep in the
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filename. If two filenames are specified, only 2 restart files will
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be created, with those names. LAMMPS will toggle between the 2 names
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as it writes successive restart files.</p>
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<p>Note that you can specify the restart command twice, once with a
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single filename and once with two filenames. This would allow you,
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for example, to write out archival restart files every 100000 steps
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using a single filenname, and more frequent temporary restart files
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every 1000 steps, using two filenames. Using restart 0 will turn off
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both modes of output.</p>
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<p>Similar to <a class="reference internal" href="dump.html"><span class="doc">dump</span></a> files, the restart filename(s) can contain
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two wild-card characters.</p>
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<p>If a “*” appears in the single filename, it is replaced with the
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current timestep value. This is only recognized when a single
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filename is used (not when toggling back and forth). Thus, the 3rd
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example above creates restart files as follows: restart.1000.equil,
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restart.2000.equil, etc. If a single filename is used with no “*”,
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then the timestep value is appended. E.g. the 2nd example above
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creates restart files as follows: poly.restart.1000,
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poly.restart.2000, etc.</p>
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<p>If a “%” character appears in the restart filename(s), then one file
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is written for each processor and the “%” character is replaced with
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the processor ID from 0 to P-1. An additional file with the “%”
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replaced by “base” is also written, which contains global information.
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For example, the files written on step 1000 for filename restart.%
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would be restart.base.1000, restart.0.1000, restart.1.1000, ...,
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restart.P-1.1000. This creates smaller files and can be a fast mode
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of output and subsequent input on parallel machines that support
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parallel I/O. The optional <em>fileper</em> and <em>nfile</em> keywords discussed
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below can alter the number of files written.</p>
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<p>The restart file can also be written in parallel as one large binary
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file via the MPI-IO library, which is part of the MPI standard for
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versions 2.0 and above. Using MPI-IO requires two steps. First,
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build LAMMPS with its MPIIO package installed, e.g.</p>
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<div class="highlight-default"><div class="highlight"><pre><span></span><span class="n">make</span> <span class="n">yes</span><span class="o">-</span><span class="n">mpiio</span> <span class="c1"># installs the MPIIO package</span>
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<span class="n">make</span> <span class="n">g</span><span class="o">++</span> <span class="c1"># build LAMMPS for your platform</span>
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</pre></div>
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</div>
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<p>Second, use a restart filename which contains ”.mpiio”. Note that it
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does not have to end in ”.mpiio”, just contain those characters.
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Unlike MPI-IO dump files, a particular restart file must be both
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written and read using MPI-IO.</p>
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<p>Restart files are written on timesteps that are a multiple of N but
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not on the first timestep of a run or minimization. You can use the
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<a class="reference internal" href="write_restart.html"><span class="doc">write_restart</span></a> command to write a restart file
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before a run begins. A restart file is not written on the last
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timestep of a run unless it is a multiple of N. A restart file is
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written on the last timestep of a minimization if N > 0 and the
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minimization converges.</p>
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<p>Instead of a numeric value, N can be specifed as an <a class="reference internal" href="variable.html"><span class="doc">equal-style variable</span></a>, which should be specified as v_name, where
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name is the variable name. In this case, the variable is evaluated at
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the beginning of a run to determine the next timestep at which a
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restart file will be written out. On that timestep, the variable will
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be evaluated again to determine the next timestep, etc. Thus the
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variable should return timestep values. See the stagger() and
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logfreq() and stride() math functions for <a class="reference internal" href="variable.html"><span class="doc">equal-style variables</span></a>, as examples of useful functions to use in
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this context. Other similar math functions could easily be added as
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options for <a class="reference internal" href="variable.html"><span class="doc">equal-style variables</span></a>.</p>
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<p>For example, the following commands will write restart files
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every step from 1100 to 1200, and could be useful for debugging
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a simulation where something goes wrong at step 1163:</p>
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<div class="highlight-default"><div class="highlight"><pre><span></span><span class="n">variable</span> <span class="n">s</span> <span class="n">equal</span> <span class="n">stride</span><span class="p">(</span><span class="mi">1100</span><span class="p">,</span><span class="mi">1200</span><span class="p">,</span><span class="mi">1</span><span class="p">)</span>
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<span class="n">restart</span> <span class="n">v_s</span> <span class="n">tmp</span><span class="o">.</span><span class="n">restart</span>
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</pre></div>
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</div>
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<hr class="docutils" />
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<p>See the <a class="reference internal" href="read_restart.html"><span class="doc">read_restart</span></a> command for information about
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what is stored in a restart file.</p>
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<p>Restart files can be read by a <a class="reference internal" href="read_restart.html"><span class="doc">read_restart</span></a>
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command to restart a simulation from a particular state. Because the
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file is binary (to enable exact restarts), it may not be readable on
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another machine. In this case, you can use the <a class="reference internal" href="Section_start.html#start-7"><span class="std std-ref">-r command-line switch</span></a> to convert a restart file to a data
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file.</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">Although the purpose of restart files is to enable restarting a
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simulation from where it left off, not all information about a
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simulation is stored in the file. For example, the list of fixes that
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were specified during the initial run is not stored, which means the
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new input script must specify any fixes you want to use. Even when
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restart information is stored in the file, as it is for some fixes,
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commands may need to be re-specified in the new input script, in order
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to re-use that information. See the <a class="reference internal" href="read_restart.html"><span class="doc">read_restart</span></a>
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command for information about what is stored in a restart file.</p>
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</div>
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<hr class="docutils" />
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<p>The optional <em>nfile</em> or <em>fileper</em> keywords can be used in conjunction
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with the “%” wildcard character in the specified restart file name(s).
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As explained above, the “%” character causes the restart file to be
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written in pieces, one piece for each of P processors. By default P =
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the number of processors the simulation is running on. The <em>nfile</em> or
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<em>fileper</em> keyword can be used to set P to a smaller value, which can
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be more efficient when running on a large number of processors.</p>
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<p>The <em>nfile</em> keyword sets P to the specified Nf value. For example, if
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Nf = 4, and the simulation is running on 100 processors, 4 files will
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be written, by processors 0,25,50,75. Each will collect information
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from itself and the next 24 processors and write it to a restart file.</p>
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<p>For the <em>fileper</em> keyword, the specified value of Np means write one
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file for every Np processors. For example, if Np = 4, every 4th
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processor (0,4,8,12,etc) will collect information from itself and the
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next 3 processors and write it to a restart file.</p>
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</div>
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<hr class="docutils" />
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<div class="section" id="restrictions">
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<h2>Restrictions</h2>
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<p>To write and read restart files in parallel with MPI-IO, the MPIIO
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package must be installed.</p>
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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="write_restart.html"><span class="doc">write_restart</span></a>, <a class="reference internal" href="read_restart.html"><span class="doc">read_restart</span></a></p>
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</div>
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<div class="section" id="default">
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<h2>Default</h2>
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<div class="highlight-default"><div class="highlight"><pre><span></span><span class="n">restart</span> <span class="mi">0</span>
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</pre></div>
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