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<li class="toctree-l1"><a class="reference internal" href="Section_intro.html">1. Introduction</a></li>
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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>fix recenter command</li>
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<div class="section" id="fix-recenter-command">
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<span id="index-0"></span><h1>fix recenter 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">recenter</span> <span class="n">x</span> <span class="n">y</span> <span class="n">z</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>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>recenter = style name of this fix command</li>
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<li>x,y,z = constrain center-of-mass to these coords (distance units), any coord can also be NULL or INIT (see below)</li>
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<li>zero or more keyword/value pairs may be appended</li>
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<li>keyword = <em>shift</em> or <em>units</em></li>
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</ul>
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<pre class="literal-block">
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<em>shift</em> value = group-ID
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group-ID = group of atoms whose coords are shifted
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<em>units</em> value = <em>box</em> or <em>lattice</em> or <em>fraction</em>
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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">fix</span> <span class="mi">1</span> <span class="nb">all</span> <span class="n">recenter</span> <span class="mf">0.0</span> <span class="mf">0.5</span> <span class="mf">0.0</span>
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<span class="n">fix</span> <span class="mi">1</span> <span class="nb">all</span> <span class="n">recenter</span> <span class="n">INIT</span> <span class="n">INIT</span> <span class="n">NULL</span>
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<span class="n">fix</span> <span class="mi">1</span> <span class="nb">all</span> <span class="n">recenter</span> <span class="n">INIT</span> <span class="mf">0.0</span> <span class="mf">0.0</span> <span class="n">units</span> <span class="n">box</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>Constrain the center-of-mass position of a group of atoms by adjusting
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the coordinates of the atoms every timestep. This is simply a small
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shift that does not alter the dynamics of the system or change the
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relative coordinates of any pair of atoms in the group. This can be
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used to insure the entire collection of atoms (or a portion of them)
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do not drift during the simulation due to random perturbations
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(e.g. <a class="reference internal" href="fix_langevin.html"><span class="doc">fix langevin</span></a> thermostatting).</p>
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<p>Distance units for the x,y,z values are determined by the setting of
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the <em>units</em> keyword, as discussed below. One or more x,y,z values can
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also be specified as NULL, which means exclude that dimension from
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this operation. Or it can be specified as INIT which means to
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constrain the center-of-mass to its initial value at the beginning of
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the run.</p>
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<p>The center-of-mass (COM) is computed for the group specified by the
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fix. If the current COM is different than the specified x,y,z, then a
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group of atoms has their coordinates shifted by the difference. By
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default the shifted group is also the group specified by the fix. A
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different group can be shifted by using the <em>shift</em> keyword. For
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example, the COM could be computed on a protein to keep it in the
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center of the simulation box. But the entire system (protein + water)
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could be shifted.</p>
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<p>If the <em>units</em> keyword is set to <em>box</em>, then the distance units of
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x,y,z are defined by the <a class="reference internal" href="units.html"><span class="doc">units</span></a> command - e.g. Angstroms
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for <em>real</em> units. A <em>lattice</em> value means the distance units are in
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lattice spacings. The <a class="reference internal" href="lattice.html"><span class="doc">lattice</span></a> command must have been
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previously used to define the lattice spacing. A <em>fraction</em> value
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means a fractional distance between the lo/hi box boundaries, e.g. 0.5
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= middle of the box. The default is to use lattice units.</p>
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<p>Note that the <a class="reference internal" href="velocity.html"><span class="doc">velocity</span></a> command can be used to create
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velocities with zero aggregate linear and/or angular momentum.</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">This fix performs its operations at the same point in the
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timestep as other time integration fixes, such as <a class="reference internal" href="fix_nve.html"><span class="doc">fix nve</span></a>, <a class="reference internal" href="fix_nh.html"><span class="doc">fix nvt</span></a>, or <a class="reference internal" href="fix_nh.html"><span class="doc">fix npt</span></a>.
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Thus fix recenter should normally be the last such fix specified in
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the input script, since the adjustments it makes to atom coordinates
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should come after the changes made by time integration. LAMMPS will
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warn you if your fixes are not ordered this way.</p>
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</div>
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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">If you use this fix on a small group of atoms (e.g. a molecule
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in solvent) without using the <em>shift</em> keyword to adjust the positions
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of all atoms in the system, then the results can be unpredictable.
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For example, if the molecule is pushed consistently in one direction
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by a flowing solvent, its velocity will increase. But its coordinates
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will be recentered, meaning it is moved back towards the force. Thus
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over time, the velocity and effective temperature of the molecule
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could become very large, though it won’t actually be moving due to the
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recentering. If you are thermostatting the entire system, then the
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solvent would be cooled to compensate. A better solution for this
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simulation scenario is to use the <a class="reference internal" href="fix_spring.html"><span class="doc">fix spring</span></a> command
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to tether the molecule in place.</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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distance the group is moved by fix recenter.</p>
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<p>This fix also computes global 3-vector which can be accessed by
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various <a class="reference internal" href="Section_howto.html#howto-15"><span class="std std-ref">output commands</span></a>. The 3
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quantities in the vector are xyz components of displacement applied to
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the group of atoms by the fix.</p>
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<p>The scalar and vector values calculated by this fix are “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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<p>This fix should not be used with an x,y,z setting that causes a large
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shift in the system on the 1st timestep, due to the requested COM
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being very different from the initial COM. This could cause atoms to
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be lost, especially in parallel. Instead, use the
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<a class="reference internal" href="displace_atoms.html"><span class="doc">displace_atoms</span></a> command, which can be used to
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move atoms a large distance.</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="fix_momentum.html"><span class="doc">fix momentum</span></a>, <a class="reference internal" href="velocity.html"><span class="doc">velocity</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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<p>The option defaults are shift = fix group-ID, and units = lattice.</p>
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