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<title>fix aveforce command — LAMMPS 15 May 2015 version documentation</title>
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<div class="section" id="fix-aveforce-command">
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<span id="index-0"></span><h1>fix aveforce command<a class="headerlink" href="#fix-aveforce-command" title="Permalink to this headline">¶</a></h1>
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<div class="section" id="fix-aveforce-cuda-command">
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<h1>fix aveforce/cuda command<a class="headerlink" href="#fix-aveforce-cuda-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>fix ID group-ID aveforce fx fy fz 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>ID, group-ID are documented in <a class="reference internal" href="fix.html"><em>fix</em></a> command</li>
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<li>aveforce = style name of this fix command</li>
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<li>fx,fy,fz = force component values (force units)</li>
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</ul>
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<div class="highlight-python"><div class="highlight"><pre>any of fx,fy,fz can be a variable (see below)
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</pre></div>
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</div>
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<ul class="simple">
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<li>zero or more keyword/value pairs may be appended to args</li>
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<li>keyword = <em>region</em></li>
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</ul>
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<pre class="literal-block">
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<em>region</em> value = region-ID
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region-ID = ID of region atoms must be in to have added force
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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>fix pressdown topwall aveforce 0.0 -1.0 0.0
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fix 2 bottomwall aveforce NULL -1.0 0.0 region top
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fix 2 bottomwall aveforce NULL -1.0 v_oscillate region top
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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>Apply an additional external force to a group of atoms in such a way
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that every atom experiences the same force. This is useful for
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pushing on wall or boundary atoms so that the structure of the wall
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does not change over time.</p>
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<p>The existing force is averaged for the group of atoms, component by
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component. The actual force on each atom is then set to the average
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value plus the component specified in this command. This means each
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atom in the group receives the same force.</p>
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<p>Any of the fx,fy,fz values can be specified as NULL which means the
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force in that dimension is not changed. Note that this is not the
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same as specifying a 0.0 value, since that sets all forces to the same
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average value without adding in any additional force.</p>
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<p>Any of the 3 quantities defining the force components can be specified
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as an equal-style <a class="reference internal" href="variable.html"><em>variable</em></a>, namely <em>fx</em>, <em>fy</em>, <em>fz</em>.
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If the value is a variable, it should be specified as v_name, where
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name is the variable name. In this case, the variable will be
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evaluated each timestep, and its value used to determine the average
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force.</p>
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<p>Equal-style variables can specify formulas with various mathematical
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functions, and include <a class="reference internal" href="thermo_style.html"><em>thermo_style</em></a> command
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keywords for the simulation box parameters and timestep and elapsed
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time. Thus it is easy to specify a time-dependent average force.</p>
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<p>If the <em>region</em> keyword is used, the atom must also be in the
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specified geometric <a class="reference internal" href="region.html"><em>region</em></a> in order to have force added
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to it.</p>
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<hr class="docutils" />
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<p>Styles with a <em>cuda</em> suffix are functionally the same as the
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corresponding style without the suffix. They have been optimized to
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run faster, depending on your available hardware, as discussed in
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<a class="reference internal" href="Section_accelerate.html"><em>Section_accelerate</em></a> of the manual. The
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accelerated styles take the same arguments and should produce the same
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results, except for round-off and precision issues.</p>
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<p>These accelerated styles are part of the USER-CUDA package. They are
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only enabled if LAMMPS was built with that package. See the <a class="reference internal" href="Section_start.html#start-3"><span>Making LAMMPS</span></a> section for more info.</p>
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<p>You can specify the accelerated styles explicitly in your input script
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by including their suffix, or you can use the <a class="reference internal" href="Section_start.html#start-7"><span>-suffix command-line switch</span></a> when you invoke LAMMPS, or you can
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use the <a class="reference internal" href="suffix.html"><em>suffix</em></a> command in your input script.</p>
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<p>See <a class="reference internal" href="Section_accelerate.html"><em>Section_accelerate</em></a> of the manual for
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more instructions on how to use the accelerated styles effectively.</p>
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</div>
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<hr class="docutils" />
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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<a class="headerlink" href="#restart-fix-modify-output-run-start-stop-minimize-info" title="Permalink to this headline">¶</a></h2>
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<p>No information about this fix is written to <a class="reference internal" href="restart.html"><em>binary restart files</em></a>. None of the <a class="reference internal" href="fix_modify.html"><em>fix_modify</em></a> options
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are relevant to this fix.</p>
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<p>This fix computes a global 3-vector of forces, which can be accessed
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by various <a class="reference internal" href="Section_howto.html#howto-15"><span>output commands</span></a>. This is the
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total force on the group of atoms before the forces on individual
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atoms are changed by the fix. The vector values calculated by this
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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"><em>run</em></a> command.</p>
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<p>The forces due to this fix are imposed during an energy minimization,
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invoked by the <a class="reference internal" href="minimize.html"><em>minimize</em></a> command. You should not
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specify force components with a variable that has time-dependence for
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use with a minimizer, since the minimizer increments the timestep as
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the iteration count during the minimization.</p>
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</div>
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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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<blockquote>
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<div>none</div></blockquote>
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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="fix_setforce.html"><em>fix setforce</em></a>, <a class="reference internal" href="fix_addforce.html"><em>fix addforce</em></a></p>
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<p><strong>Default:</strong> none</p>
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