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<div class="section" id="fix-vector-command">
<span id="index-0"></span><h1>fix vector command<a class="headerlink" href="#fix-vector-command" title="Permalink to this headline"></a></h1>
<div class="section" id="syntax">
<h2>Syntax<a class="headerlink" href="#syntax" title="Permalink to this headline"></a></h2>
<div class="highlight-python"><div class="highlight"><pre>fix ID group-ID vector Nevery value1 value2 ...
</pre></div>
</div>
<ul class="simple">
<li>ID, group-ID are documented in <a class="reference internal" href="fix.html"><em>fix</em></a> command</li>
<li>vector = style name of this fix command</li>
<li>Nevery = use input values every this many timesteps</li>
<li>one or more input values can be listed</li>
<li>value = c_ID, c_ID[N], f_ID, f_ID[N], v_name</li>
</ul>
<div class="highlight-python"><div class="highlight"><pre>c_ID = global scalar calculated by a compute with ID
c_ID[I] = Ith component of global vector calculated by a compute with ID
f_ID = global scalar calculated by a fix with ID
f_ID[I] = Ith component of global vector calculated by a fix with ID
v_name = value calculated by an equal-style variable with name
v_name[I] = Ith component of vector-style variable with name
</pre></div>
</div>
</div>
<div class="section" id="examples">
<h2>Examples<a class="headerlink" href="#examples" title="Permalink to this headline"></a></h2>
<div class="highlight-python"><div class="highlight"><pre>fix 1 all vector 100 c_myTemp
fix 1 all vector 5 c_myTemp v_integral
</pre></div>
</div>
</div>
<div class="section" id="description">
<h2>Description<a class="headerlink" href="#description" title="Permalink to this headline"></a></h2>
<p>Use one or more global values as inputs every few timesteps, and
simply store them. For a single specified value, the values are
stored as a global vector of growing length. For multiple specified
values, they are stored as rows in a global array, whose number of
rows is growing. The resulting vector or array can be used by other
<a class="reference internal" href="Section_howto.html#howto-15"><span>output commands</span></a>.</p>
<p>One way to to use this command is to accumulate a vector that is
time-integrated using the <a class="reference internal" href="variable.html"><em>variable trap()</em></a> function.
For example the velocity auto-correlation function (VACF) can be
time-integrated, to yield a diffusion coefficient, as follows:</p>
<div class="highlight-python"><div class="highlight"><pre>compute 2 all vacf
fix 5 all vector 1 c_2[4]
variable diff equal dt*trap(f_5)
thermo_style custom step v_diff
</pre></div>
</div>
<p>The group specified with this command is ignored. However, note that
specified values may represent calculations performed by computes and
fixes which store their own &#8220;group&#8221; definitions.</p>
<p>Each listed value can be the result of a <a class="reference internal" href="compute.html"><em>compute</em></a> or
<a class="reference internal" href="fix.html"><em>fix</em></a> or the evaluation of an equal-style or vector-style
<a class="reference internal" href="variable.html"><em>variable</em></a>. In each case, the compute, fix, or variable
must produce a global quantity, not a per-atom or local quantity. And
the global quantity must be a scalar, not a vector or array.</p>
<p><a class="reference internal" href="compute.html"><em>Computes</em></a> that produce global quantities are those which
do not have the word <em>atom</em> in their style name. Only a few
<a class="reference internal" href="fix.html"><em>fixes</em></a> produce global quantities. See the doc pages for
individual fixes for info on which ones produce such values.
<a class="reference internal" href="variable.html"><em>Variables</em></a> of style <em>equal</em> or <em>vector</em> are the only
ones that can be used with this fix. Variables of style <em>atom</em> cannot
be used, since they produce per-atom values.</p>
<p>The <em>Nevery</em> argument specifies on what timesteps the input values
will be used in order to be stored. Only timesteps that are a
multiple of <em>Nevery</em>, including timestep 0, will contribute values.</p>
<p>Note that if you perform multiple runs, using the &#8220;pre no&#8221; option of
the <a class="reference internal" href="run.html"><em>run</em></a> command to avoid initialization on subsequent runs,
then you need to use the <em>stop</em> keyword with the first <a class="reference internal" href="run.html"><em>run</em></a>
command with a timestep value that encompasses all the runs. This is
so that the vector or array stored by this fix can be allocated to a
sufficient size.</p>
<hr class="docutils" />
<p>If a value begins with &#8220;<a href="#id1"><span class="problematic" id="id2">c_</span></a>&#8221;, a compute ID must follow which has been
previously defined in the input script. If no bracketed term is
appended, the global scalar calculated by the compute is used. If a
bracketed term is appended, the Ith element of the global vector
calculated by the compute is used.</p>
<p>Note that there is a <a class="reference internal" href="compute_reduce.html"><em>compute reduce</em></a> command
which can sum per-atom quantities into a global scalar or vector which
can thus be accessed by fix vector. Or it can be a compute defined
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>
or <a class="reference internal" href="fix_temp_rescale.html"><em>fix temp/rescale</em></a>. See the doc pages for
these commands which give the IDs of these computes. Users can also
write code for their own compute styles and <a class="reference internal" href="Section_modify.html"><em>add them to LAMMPS</em></a>.</p>
<p>If a value begins with &#8220;<a href="#id3"><span class="problematic" id="id4">f_</span></a>&#8221;, a fix ID must follow which has been
previously defined in the input script. If no bracketed term is
appended, the global scalar calculated by the fix is used. If a
bracketed term is appended, the Ith element of the global vector
calculated by the fix is used.</p>
<p>Note that some fixes only produce their values on certain timesteps,
which must be compatible with <em>Nevery</em>, else an error will result.
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>
<p>If a value begins with &#8220;<a href="#id5"><span class="problematic" id="id6">v_</span></a>&#8221;, a variable name must follow which has
been previously defined in the input script. An equal-style or
vector-style variable can be referenced; the latter requires a
bracketed term to use the Ith element of the vector calculated by the
variable. See the <a class="reference internal" href="variable.html"><em>variable</em></a> command for details. Note
that variables of style <em>equal</em> and <em>vector</em> define a formula which
can reference individual atom properties or thermodynamic keywords, or
they can invoke other computes, fixes, or variables when they are
evaluated, so this is a very general means of specifying quantities to
be stored by fix vector.</p>
</div>
<hr class="docutils" />
<div class="section" id="restart-fix-modify-output-run-start-stop-minimize-info">
<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>
<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
are relevant to this fix.</p>
<p>This fix produces a global vector or global array which can be
accessed by various <a class="reference internal" href="Section_howto.html#howto-15"><span>output commands</span></a>.
The values can only be accessed on timesteps that are multiples of
<em>Nevery</em>.</p>
<p>A vector is produced if only a single input value is specified.
An array is produced if multiple input values are specified.
The length of the vector or the number of rows in the array grows
by 1 every <em>Nevery</em> timesteps.</p>
<p>If the fix prouduces a vector, then the entire vector will be either
&#8220;intensive&#8221; or &#8220;extensive&#8221;, depending on whether the values stored in
the vector are &#8220;intensive&#8221; or &#8220;extensive&#8221;. If the fix produces an
array, then all elements in the array must be the same, either
&#8220;intensive&#8221; or &#8220;extensive&#8221;. If a compute or fix provides the value
stored, then the compute or fix determines whether the value is
intensive or extensive; see the doc page for that compute or fix for
further info. Values produced by a variable are treated as intensive.</p>
<p>This fix can allocate storage for stored values accumulated over
multiple runs, using the <em>start</em> and <em>stop</em> keywords of the
<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
how to do this. If using the <a class="reference internal" href="run.html"><em>run pre no</em></a> command option,
this is required to allow the fix to allocate sufficient storage for
stored values.</p>
<p>This fix is not invoked during <a class="reference internal" href="minimize.html"><em>energy minimization</em></a>.</p>
</div>
<div class="section" id="restrictions">
<h2>Restrictions<a class="headerlink" href="#restrictions" title="Permalink to this headline"></a></h2>
<blockquote>
<div>none</div></blockquote>
</div>
<div class="section" id="related-commands">
<h2>Related commands<a class="headerlink" href="#related-commands" title="Permalink to this headline"></a></h2>
<p><a class="reference internal" href="compute.html"><em>compute</em></a>, <a class="reference internal" href="variable.html"><em>variable</em></a></p>
<p><strong>Default:</strong> none</p>
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