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<div class="section" id="fix-vector-command">
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<span id="index-0"></span><h1>fix vector command<a class="headerlink" href="#fix-vector-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 vector Nevery value1 value2 ...
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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>vector = style name of this fix command</li>
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<li>Nevery = use input values every this many timesteps</li>
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<li>one or more input values can be listed</li>
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<li>value = c_ID, c_ID[N], f_ID, f_ID[N], v_name</li>
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</ul>
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<div class="highlight-python"><div class="highlight"><pre>c_ID = global scalar calculated by a compute with ID
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c_ID[I] = Ith component of global vector calculated by a compute with ID
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f_ID = global scalar calculated by a fix with ID
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f_ID[I] = Ith component of global vector calculated by a fix with ID
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v_name = global value calculated by an equal-style variable with name
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</pre></div>
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</div>
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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 1 all vector 100 c_myTemp
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fix 1 all vector 5 c_myTemp v_integral
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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>Use one or more global values as inputs every few timesteps, and
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simply store them. For a single specified value, the values are
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stored as a global vector of growing length. For multiple specified
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values, they are stored as rows in a global array, whose number of
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rows is growing. The resulting vector or array can be used by other
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<a class="reference internal" href="Section_howto.html#howto-15"><span>output commands</span></a>.</p>
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<p>One way to to use this command is to accumulate a vector that is
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time-integrated using the <a class="reference internal" href="variable.html"><em>variable trap()</em></a> function.
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For example the velocity auto-correlation function (VACF) can be
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time-integrated, to yield a diffusion coefficient, as follows:</p>
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<div class="highlight-python"><div class="highlight"><pre>compute 2 all vacf
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fix 5 all vector 1 c_2[4]
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variable diff equal dt*trap(f_5)
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thermo_style custom step v_diff
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</pre></div>
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</div>
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<p>The group specified with this command is ignored. However, note that
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specified values may represent calculations performed by computes and
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fixes which store their own “group” definitions.</p>
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<p>Each listed value can be the result of a <a class="reference internal" href="compute.html"><em>compute</em></a> or
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<a class="reference internal" href="fix.html"><em>fix</em></a> or the evaluation of an equal-style
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<a class="reference internal" href="variable.html"><em>variable</em></a>. In each case, the compute, fix, or variable
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must produce a global quantity, not a per-atom or local quantity.
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And the global quantity must be a scalar, not a vector or array.</p>
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<p><a class="reference internal" href="compute.html"><em>Computes</em></a> that produce global quantities are those which
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do not have the word <em>atom</em> in their style name. Only a few
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<a class="reference internal" href="fix.html"><em>fixes</em></a> produce global quantities. See the doc pages for
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individual fixes for info on which ones produce such values.
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<a class="reference internal" href="variable.html"><em>Variables</em></a> of style <em>equal</em> are the only ones that can
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be used with this fix. Variables of style <em>atom</em> cannot be used,
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since they produce per-atom values.</p>
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<p>The <em>Nevery</em> argument specifies on what timesteps the input values
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will be used in order to be stored. Only timesteps that are a
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multiple of <em>Nevery</em>, including timestep 0, will contribute values.</p>
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<p>Note that if you perform multiple runs, using the “pre no” option of
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the <a class="reference internal" href="run.html"><em>run</em></a> command to avoid initialization on subsequent runs,
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then you need to use the <em>stop</em> keyword with the first <a class="reference internal" href="run.html"><em>run</em></a>
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command with a timestep value that encompasses all the runs. This is
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so that the vector or array stored by this fix can be allocated to a
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sufficient size.</p>
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<hr class="docutils" />
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<p>If a value begins with “<a href="#id1"><span class="problematic" id="id2">c_</span></a>”, a compute ID must follow which has been
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previously defined in the input script. If no bracketed term is
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appended, the global scalar calculated by the compute is used. If a
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bracketed term is appended, the Ith element of the global vector
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calculated by the compute is used.</p>
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<p>Note that there is a <a class="reference internal" href="compute_reduce.html"><em>compute reduce</em></a> command
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which can sum per-atom quantities into a global scalar or vector which
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can thus be accessed by fix vector. Or it can be a compute defined
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not in your input script, but by <a class="reference internal" href="thermo_style.html"><em>thermodynamic output</em></a> or other fixes such as <a class="reference internal" href="fix_nh.html"><em>fix nvt</em></a>
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or <a class="reference internal" href="fix_temp_rescale.html"><em>fix temp/rescale</em></a>. See the doc pages for
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these commands which give the IDs of these computes. Users can also
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write code for their own compute styles and <a class="reference internal" href="Section_modify.html"><em>add them to LAMMPS</em></a>.</p>
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<p>If a value begins with “<a href="#id3"><span class="problematic" id="id4">f_</span></a>”, a fix ID must follow which has been
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previously defined in the input script. If no bracketed term is
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appended, the global scalar calculated by the fix is used. If a
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bracketed term is appended, the Ith element of the global vector
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calculated by the fix is used.</p>
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<p>Note that some fixes only produce their values on certain timesteps,
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which must be compatible with <em>Nevery</em>, else an error will result.
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Users can also write code for their own fix styles and <a class="reference internal" href="Section_modify.html"><em>add them to LAMMPS</em></a>.</p>
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<p>If a value begins with “<a href="#id5"><span class="problematic" id="id6">v_</span></a>”, a variable name must follow which has
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been previously defined in the input script. Only equal-style
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variables can be referenced. See the <a class="reference internal" href="variable.html"><em>variable</em></a> command
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for details. Note that variables of style <em>equal</em> define a formula
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which can reference individual atom properties or thermodynamic
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keywords, or they can invoke other computes, fixes, or variables when
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they are evaluated, so this is a very general means of specifying
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quantities to be stored by fix vector.</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 produces a global vector or global array which can be
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accessed by various <a class="reference internal" href="Section_howto.html#howto-15"><span>output commands</span></a>.
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The values can only be accessed on timesteps that are multiples of
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<em>Nevery</em>.</p>
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<p>A vector is produced if only a single input value is specified.
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An array is produced if multiple input values are specified.
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The length of the vector or the number of rows in the array grows
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by 1 every <em>Nevery</em> timesteps.</p>
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<p>If the fix prouduces a vector, then the entire vector will be either
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“intensive” or “extensive”, depending on whether the values stored in
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the vector are “intensive” or “extensive”. If the fix produces an
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array, then all elements in the array must be the same, either
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“intensive” or “extensive”. If a compute or fix provides the value
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stored, then the compute or fix determines whether the value is
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intensive or extensive; see the doc page for that compute or fix for
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further info. Values produced by a variable are treated as intensive.</p>
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<p>This fix can allocate storage for stored values accumulated over
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multiple runs, using the <em>start</em> and <em>stop</em> keywords of the
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<a class="reference internal" href="run.html"><em>run</em></a> command. See the <a class="reference internal" href="run.html"><em>run</em></a> command for details of
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how to do this. If using the <a class="reference internal" href="run.html"><em>run pre no</em></a> command option,
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this is required to allow the fix to allocate sufficient storage for
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stored values.</p>
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<p>This fix is not invoked during <a class="reference internal" href="minimize.html"><em>energy minimization</em></a>.</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="compute.html"><em>compute</em></a>, <a class="reference internal" href="variable.html"><em>variable</em></a></p>
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<p><strong>Default:</strong> none</p>
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