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<div class="section" id="fix-ave-atom-command">
<span id="index-0"></span><h1>fix ave/atom command</h1>
<div class="section" id="syntax">
<h2>Syntax</h2>
<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">ave</span><span class="o">/</span><span class="n">atom</span> <span class="n">Nevery</span> <span class="n">Nrepeat</span> <span class="n">Nfreq</span> <span class="n">value1</span> <span class="n">value2</span> <span class="o">...</span>
</pre></div>
</div>
<ul class="simple">
<li>ID, group-ID are documented in <a class="reference internal" href="fix.html"><span class="doc">fix</span></a> command</li>
<li>ave/atom = style name of this fix command</li>
<li>Nevery = use input values every this many timesteps</li>
<li>Nrepeat = # of times to use input values for calculating averages</li>
<li>Nfreq = calculate averages every this many timesteps
one or more input values can be listed</li>
<li>value = x, y, z, vx, vy, vz, fx, fy, fz, c_ID, c_ID[i], f_ID, f_ID[i], v_name</li>
</ul>
<div class="highlight-default"><div class="highlight"><pre><span></span><span class="n">x</span><span class="p">,</span><span class="n">y</span><span class="p">,</span><span class="n">z</span><span class="p">,</span><span class="n">vx</span><span class="p">,</span><span class="n">vy</span><span class="p">,</span><span class="n">vz</span><span class="p">,</span><span class="n">fx</span><span class="p">,</span><span class="n">fy</span><span class="p">,</span><span class="n">fz</span> <span class="o">=</span> <span class="n">atom</span> <span class="n">attribute</span> <span class="p">(</span><span class="n">position</span><span class="p">,</span> <span class="n">velocity</span><span class="p">,</span> <span class="n">force</span> <span class="n">component</span><span class="p">)</span>
<span class="n">c_ID</span> <span class="o">=</span> <span class="n">per</span><span class="o">-</span><span class="n">atom</span> <span class="n">vector</span> <span class="n">calculated</span> <span class="n">by</span> <span class="n">a</span> <span class="n">compute</span> <span class="k">with</span> <span class="n">ID</span>
<span class="n">c_ID</span><span class="p">[</span><span class="n">I</span><span class="p">]</span> <span class="o">=</span> <span class="n">Ith</span> <span class="n">column</span> <span class="n">of</span> <span class="n">per</span><span class="o">-</span><span class="n">atom</span> <span class="n">array</span> <span class="n">calculated</span> <span class="n">by</span> <span class="n">a</span> <span class="n">compute</span> <span class="k">with</span> <span class="n">ID</span><span class="p">,</span> <span class="n">I</span> <span class="n">can</span> <span class="n">include</span> <span class="n">wildcard</span> <span class="p">(</span><span class="n">see</span> <span class="n">below</span><span class="p">)</span>
<span class="n">f_ID</span> <span class="o">=</span> <span class="n">per</span><span class="o">-</span><span class="n">atom</span> <span class="n">vector</span> <span class="n">calculated</span> <span class="n">by</span> <span class="n">a</span> <span class="n">fix</span> <span class="k">with</span> <span class="n">ID</span>
<span class="n">f_ID</span><span class="p">[</span><span class="n">I</span><span class="p">]</span> <span class="o">=</span> <span class="n">Ith</span> <span class="n">column</span> <span class="n">of</span> <span class="n">per</span><span class="o">-</span><span class="n">atom</span> <span class="n">array</span> <span class="n">calculated</span> <span class="n">by</span> <span class="n">a</span> <span class="n">fix</span> <span class="k">with</span> <span class="n">ID</span><span class="p">,</span> <span class="n">I</span> <span class="n">can</span> <span class="n">include</span> <span class="n">wildcard</span> <span class="p">(</span><span class="n">see</span> <span class="n">below</span><span class="p">)</span>
<span class="n">v_name</span> <span class="o">=</span> <span class="n">per</span><span class="o">-</span><span class="n">atom</span> <span class="n">vector</span> <span class="n">calculated</span> <span class="n">by</span> <span class="n">an</span> <span class="n">atom</span><span class="o">-</span><span class="n">style</span> <span class="n">variable</span> <span class="k">with</span> <span class="n">name</span>
</pre></div>
</div>
</div>
<div class="section" id="examples">
<h2>Examples</h2>
<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">ave</span><span class="o">/</span><span class="n">atom</span> <span class="mi">1</span> <span class="mi">100</span> <span class="mi">100</span> <span class="n">vx</span> <span class="n">vy</span> <span class="n">vz</span>
<span class="n">fix</span> <span class="mi">1</span> <span class="nb">all</span> <span class="n">ave</span><span class="o">/</span><span class="n">atom</span> <span class="mi">10</span> <span class="mi">20</span> <span class="mi">1000</span> <span class="n">c_my_stress</span><span class="p">[</span><span class="mi">1</span><span class="p">]</span>
<span class="n">fix</span> <span class="mi">1</span> <span class="nb">all</span> <span class="n">ave</span><span class="o">/</span><span class="n">atom</span> <span class="mi">10</span> <span class="mi">20</span> <span class="mi">1000</span> <span class="n">c_my_stress</span><span class="p">[</span><span class="o">*</span><span class="p">]</span>
</pre></div>
</div>
</div>
<div class="section" id="description">
<h2>Description</h2>
<p>Use one or more per-atom vectors as inputs every few timesteps, and
average them atom by atom over longer timescales. The resulting
per-atom averages can be used by other <a class="reference internal" href="Section_howto.html#howto-15"><span class="std std-ref">output commands</span></a> such as the <a class="reference internal" href="fix_ave_chunk.html"><span class="doc">fix ave/chunk</span></a> or <a class="reference internal" href="dump.html"><span class="doc">dump custom</span></a> commands.</p>
<p>The group specified with the command means only atoms within the group
have their averages computed. Results are set to 0.0 for atoms not in
the group.</p>
<p>Each input value can be an atom attribute (position, velocity, force
component) or can be the result of a <a class="reference internal" href="compute.html"><span class="doc">compute</span></a> or
<a class="reference internal" href="fix.html"><span class="doc">fix</span></a> or the evaluation of an atom-style
<a class="reference internal" href="variable.html"><span class="doc">variable</span></a>. In the latter cases, the compute, fix, or
variable must produce a per-atom vector, not a global quantity or
local quantity. If you wish to time-average global quantities from a
compute, fix, or variable, then see the <a class="reference internal" href="fix_ave_time.html"><span class="doc">fix ave/time</span></a> command.</p>
<p>Each per-atom value of each input vector is averaged independently.</p>
<p><a class="reference internal" href="compute.html"><span class="doc">Computes</span></a> that produce per-atom vectors or arrays are
those which have the word <em>atom</em> in their style name. See the doc
pages for individual <a class="reference internal" href="fix.html"><span class="doc">fixes</span></a> to determine which ones produce
per-atom vectors or arrays. <a class="reference internal" href="variable.html"><span class="doc">Variables</span></a> of style <em>atom</em>
are the only ones that can be used with this fix since they produce
per-atom vectors.</p>
<p>Note that for values from a compute or fix, the bracketed index I can
be specified using a wildcard asterisk with the index to effectively
specify multiple values. This takes the form &#8220;*&#8221; or &#8220;<em>n&#8221; or &#8220;n</em>&#8221; or
&#8220;m*n&#8221;. If N = the size of the vector (for <em>mode</em> = scalar) or the
number of columns in the array (for <em>mode</em> = vector), then an asterisk
with no numeric values means all indices from 1 to N. A leading
asterisk means all indices from 1 to n (inclusive). A trailing
asterisk means all indices from n to N (inclusive). A middle asterisk
means all indices from m to n (inclusive).</p>
<p>Using a wildcard is the same as if the individual columns of the array
had been listed one by one. E.g. these 2 fix ave/atom commands are
equivalent, since the <a class="reference internal" href="compute_stress_atom.html"><span class="doc">compute stress/atom</span></a>
command creates a per-atom array with 6 columns:</p>
<div class="highlight-default"><div class="highlight"><pre><span></span><span class="n">compute</span> <span class="n">my_stress</span> <span class="nb">all</span> <span class="n">stress</span><span class="o">/</span><span class="n">atom</span> <span class="n">NULL</span>
<span class="n">fix</span> <span class="mi">1</span> <span class="nb">all</span> <span class="n">ave</span><span class="o">/</span><span class="n">atom</span> <span class="mi">10</span> <span class="mi">20</span> <span class="mi">1000</span> <span class="n">c_my_stress</span><span class="p">[</span><span class="o">*</span><span class="p">]</span>
<span class="n">fix</span> <span class="mi">1</span> <span class="nb">all</span> <span class="n">ave</span><span class="o">/</span><span class="n">atom</span> <span class="mi">10</span> <span class="mi">20</span> <span class="mi">1000</span> <span class="n">c_my_stress</span><span class="p">[</span><span class="mi">1</span><span class="p">]</span> <span class="n">c_my_stress</span><span class="p">[</span><span class="mi">1</span><span class="p">]</span> <span class="o">&amp;</span>
<span class="n">c_my_stress</span><span class="p">[</span><span class="mi">3</span><span class="p">]</span> <span class="n">c_my_stress</span><span class="p">[</span><span class="mi">4</span><span class="p">]</span> <span class="o">&amp;</span>
<span class="n">c_my_stress</span><span class="p">[</span><span class="mi">5</span><span class="p">]</span> <span class="n">c_my_stress</span><span class="p">[</span><span class="mi">6</span><span class="p">]</span>
</pre></div>
</div>
<hr class="docutils" />
<p>The <em>Nevery</em>, <em>Nrepeat</em>, and <em>Nfreq</em> arguments specify on what
timesteps the input values will be used in order to contribute to the
average. The final averaged quantities are generated on timesteps
that are a multiple of <em>Nfreq</em>. The average is over <em>Nrepeat</em>
quantities, computed in the preceding portion of the simulation every
<em>Nevery</em> timesteps. <em>Nfreq</em> must be a multiple of <em>Nevery</em> and
<em>Nevery</em> must be non-zero even if <em>Nrepeat</em> is 1. Also, the timesteps
contributing to the average value cannot overlap,
i.e. Nrepeat*Nevery can not exceed Nfreq.</p>
<p>For example, if Nevery=2, Nrepeat=6, and Nfreq=100, then values on
timesteps 90,92,94,96,98,100 will be used to compute the final average
on timestep 100. Similarly for timesteps 190,192,194,196,198,200 on
timestep 200, etc.</p>
<hr class="docutils" />
<p>The atom attribute values (x,y,z,vx,vy,vz,fx,fy,fz) are
self-explanatory. Note that other atom attributes can be used as
inputs to this fix by using the <a class="reference internal" href="compute_property_atom.html"><span class="doc">compute property/atom</span></a> command and then specifying
an input value from that compute.</p>
<div class="admonition note">
<p class="first admonition-title">Note</p>
<p class="last">The x,y,z attributes are values that are re-wrapped inside the
periodic box whenever an atom crosses a periodic boundary. Thus if
you time average an atom that spends half its time on either side of
the periodic box, you will get a value in the middle of the box. If
this is not what you want, consider averaging unwrapped coordinates,
which can be provided by the <a class="reference internal" href="compute_property_atom.html"><span class="doc">compute property/atom</span></a> command via its xu,yu,zu
attributes.</p>
</div>
<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 per-atom vector calculated by the compute is used. If a
bracketed term containing an index I is appended, the Ith column of
the per-atom array calculated by the compute is used. Users can also
write code for their own compute styles and <a class="reference internal" href="Section_modify.html"><span class="doc">add them to LAMMPS</span></a>. See the discussion above for how I can
be specified with a wildcard asterisk to effectively specify multiple
values.</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 per-atom vector calculated by the fix is used. If a
bracketed term containing an index I is appended, the Ith column of
the per-atom array calculated by the fix is used. 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"><span class="doc">add them to LAMMPS</span></a>. See the discussion above for how I can
be specified with a wildcard asterisk to effectively specify multiple
values.</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 as an <a class="reference internal" href="variable.html"><span class="doc">atom-style variable</span></a> Variables of style <em>atom</em> can reference
thermodynamic keywords, or invoke other computes, fixes, or variables
when they are evaluated, so this is a very general means of generating
per-atom quantities to time average.</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</h2>
<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
are relevant to this fix. No global scalar or vector quantities are
stored by this fix for access by various <a class="reference internal" href="Section_howto.html#howto-15"><span class="std std-ref">output commands</span></a>.</p>
<p>This fix produces a per-atom vector or array which can be accessed by
various <a class="reference internal" href="Section_howto.html#howto-15"><span class="std std-ref">output commands</span></a>. A vector is
produced if only a single quantity is averaged by this fix. If two or
more quantities are averaged, then an array of values is produced.
The per-atom values can only be accessed on timesteps that are
multiples of <em>Nfreq</em> since that is when averaging is performed.</p>
<p>No parameter of this fix can be used with the <em>start/stop</em> keywords of
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>
</div>
<div class="section" id="restrictions">
<h2>Restrictions</h2>
<blockquote>
<div>none</div></blockquote>
</div>
<div class="section" id="related-commands">
<h2>Related commands</h2>
<p><a class="reference internal" href="compute.html"><span class="doc">compute</span></a>, <a class="reference internal" href="fix_ave_histo.html"><span class="doc">fix ave/histo</span></a>, <a class="reference internal" href="fix_ave_chunk.html"><span class="doc">fix ave/chunk</span></a>, <a class="reference internal" href="fix_ave_time.html"><span class="doc">fix ave/time</span></a>,
<a class="reference internal" href="variable.html"><span class="doc">variable</span></a>,</p>
<p><strong>Default:</strong> none</p>
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