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<title>write_data command — LAMMPS 15 May 2015 version documentation</title>
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<li class="toctree-l1"><a class="reference internal" href="Section_accelerate.html">5. Accelerating LAMMPS performance</a></li>
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<li>write_data command</li>
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<div class="section" id="write-data-command">
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<span id="index-0"></span><h1>write_data command<a class="headerlink" href="#write-data-command" title="Permalink to this headline">¶</a></h1>
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<div class="section" id="syntax">
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<h2>Syntax<a class="headerlink" href="#syntax" title="Permalink to this headline">¶</a></h2>
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<div class="highlight-python"><div class="highlight"><pre>write_data file keyword value ...
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</pre></div>
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</div>
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<ul class="simple">
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<li>file = name of data file to write out</li>
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<li>zero or more keyword/value pairs may be appended</li>
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<li>keyword = <em>pair</em> or <em>nocoeff</em></li>
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</ul>
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<pre class="literal-block">
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<em>nocoeff</em> = do not write out force field info
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<em>pair</em> value = <em>ii</em> or <em>ij</em>
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<em>ii</em> = write one line of pair coefficient info per atom type
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<em>ij</em> = write one line of pair coefficient info per IJ atom type pair
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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<a class="headerlink" href="#examples" title="Permalink to this headline">¶</a></h2>
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<div class="highlight-python"><div class="highlight"><pre>write_data data.polymer
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write_data data.*
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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<a class="headerlink" href="#description" title="Permalink to this headline">¶</a></h2>
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<p>Write a data file in text format of the current state of the
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simulation. Data files can be read by the <a class="reference internal" href="read_data.html"><em>read data</em></a>
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command to begin a simulation. The <a class="reference internal" href="read_data.html"><em>read_data</em></a> command
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also describes their format.</p>
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<p>Similar to <a class="reference internal" href="dump.html"><em>dump</em></a> files, the data filename can contain a “*”
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wild-card character. The “*” is replaced with the current timestep
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value.</p>
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<div class="admonition warning">
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<p class="first admonition-title">Warning</p>
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<p class="last">The write-data command is not yet fully implemented in
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two respects. First, most pair styles do not yet write their
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coefficient information into the data file. This means you will need
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to specify that information in your input script that reads the data
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file, via the <a class="reference internal" href="pair_coeff.html"><em>pair_coeff</em></a> command. Second, a few of
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the <a class="reference internal" href="atom_style.html"><em>atom styles</em></a> (body, ellipsoid, line, tri) that
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store auxiliary “bonus” information about aspherical particles, do not
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yet write the bonus info into the data file. Both these
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functionalities will be added to the write_data command later.</p>
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</div>
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<p>Because a data file is in text format, if you use a data file written
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out by this command to restart a simulation, the initial state of the
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new run will be slightly different than the final state of the old run
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(when the file was written) which was represented internally by LAMMPS
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in binary format. A new simulation which reads the data file will
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thus typically diverge from a simulation that continued in the
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original input script.</p>
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<p>If you want to do more exact restarts, using binary files, see the
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<a class="reference internal" href="restart.html"><em>restart</em></a>, <a class="reference internal" href="write_restart.html"><em>write_restart</em></a>, and
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<a class="reference internal" href="read_restart.html"><em>read_restart</em></a> commands. You can also convert
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binary restart files to text data files, after a simulation has run,
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using the <a class="reference internal" href="Section_start.html#start-7"><span>-r command-line switch</span></a>.</p>
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<div class="admonition warning">
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<p class="first admonition-title">Warning</p>
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<p class="last">Only limited information about a simulation is stored
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in a data file. For example, no information about atom
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<a class="reference internal" href="group.html"><em>groups</em></a> and <a class="reference internal" href="fix.html"><em>fixes</em></a> are stored. <a class="reference internal" href="read_restart.html"><em>Binary restart files</em></a> store more information.</p>
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</div>
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<p>Bond interactions (angle, etc) that have been turned off by the <a class="reference internal" href="fix_shake.html"><em>fix shake</em></a> or <a class="reference internal" href="delete_bonds.html"><em>delete_bonds</em></a> command will
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be written to a data file as if they are turned on. This means they
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will need to be turned off again in a new run after the data file is
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read.</p>
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<p>Bonds that are broken (e.g. by a bond-breaking potential) are not
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written to the data file. Thus these bonds will not exist when the
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data file is read.</p>
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<hr class="docutils" />
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<p>The <em>nocoeff</em> keyword requests that no force field parameters should
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be written to the data file. This can be very helpful, if one wants
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to make significant changes to the force field or if the parameters
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are read in separately anyway, e.g. from an include file.</p>
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<p>The <em>pair</em> keyword lets you specify in what format the pair
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coefficient information is written into the data file. If the value
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is specified as <em>ii</em>, then one line per atom type is written, to
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specify the coefficients for each of the I=J interactions. This means
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that no cross-interactions for I != J will be specified in the data
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file and the pair style will apply its mixing rule, as documented on
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individual <a class="reference internal" href="pair_style.html"><em>pair_style</em></a> doc pages. Of course this
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behavior can be overridden in the input script after reading the data
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file, by specifying additional <a class="reference internal" href="pair_coeff.html"><em>pair_coeff</em></a> commands
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for any desired I,J pairs.</p>
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<p>If the value is specified as <em>ij</em>, then one line of coefficients is
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written for all I,J pairs where I <= J. These coefficients will
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include any specific settings made in the input script up to that
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point. The presence of these I != J coefficients in the data file
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will effectively turn off the default mixing rule for the pair style.
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Again, the coefficient values in the data file can can be overridden
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in the input script after reading the data file, by specifying
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additional <a class="reference internal" href="pair_coeff.html"><em>pair_coeff</em></a> commands for any desired I,J
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pairs.</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<a class="headerlink" href="#restrictions" title="Permalink to this headline">¶</a></h2>
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<p>This command requires inter-processor communication to migrate atoms
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before the data file is written. This means that your system must be
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ready to perform a simulation before using this command (force fields
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setup, atom masses initialized, etc).</p>
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</div>
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<div class="section" id="related-commands">
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<h2>Related commands<a class="headerlink" href="#related-commands" title="Permalink to this headline">¶</a></h2>
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<p><a class="reference internal" href="read_data.html"><em>read_data</em></a>, <a class="reference internal" href="write_restart.html"><em>write_restart</em></a></p>
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</div>
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<div class="section" id="default">
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<h2>Default<a class="headerlink" href="#default" title="Permalink to this headline">¶</a></h2>
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<p>The option defaults are pair = ii.</p>
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