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<div class="section" id="compute-vacf-command">
<span id="index-0"></span><h1>compute vacf command</h1>
<div class="section" id="syntax">
<h2>Syntax</h2>
<div class="highlight-default"><div class="highlight"><pre><span></span><span class="n">compute</span> <span class="n">ID</span> <span class="n">group</span><span class="o">-</span><span class="n">ID</span> <span class="n">vacf</span>
</pre></div>
</div>
<ul class="simple">
<li>ID, group-ID are documented in <a class="reference internal" href="compute.html"><span class="doc">compute</span></a> command</li>
<li>vacf = style name of this compute command</li>
</ul>
</div>
<div class="section" id="examples">
<h2>Examples</h2>
<div class="highlight-default"><div class="highlight"><pre><span></span><span class="n">compute</span> <span class="mi">1</span> <span class="nb">all</span> <span class="n">vacf</span>
<span class="n">compute</span> <span class="mi">1</span> <span class="n">upper</span> <span class="n">vacf</span>
</pre></div>
</div>
</div>
<div class="section" id="description">
<h2>Description</h2>
<p>Define a computation that calculates the velocity auto-correlation
function (VACF), averaged over a group of atoms. Each atom&#8217;s
contribution to the VACF is its current velocity vector dotted into
its initial velocity vector at the time the compute was specified.</p>
<p>A vector of four quantites is calculated by this compute. The first 3
elements of the vector are vx * vx0 (and similarly for the y and z
components), summed and averaged over atoms in the group. Vx is the
current x-component of velocity for the atom, vx0 is the initial
x-component of velocity for the atom. The 4th element of the vector
is the total VACF, i.e. (vx*vx0 + vy*vy0 + vz*vz0), summed and
averaged over atoms in the group.</p>
<p>The integral of the VACF versus time is proportional to the diffusion
coefficient of the diffusing atoms. This can be computed in the
following manner, using the <a class="reference internal" href="variable.html"><span class="doc">variable trap()</span></a> function:</p>
<div class="highlight-default"><div class="highlight"><pre><span></span><span class="n">compute</span> <span class="mi">2</span> <span class="nb">all</span> <span class="n">vacf</span>
<span class="n">fix</span> <span class="mi">5</span> <span class="nb">all</span> <span class="n">vector</span> <span class="mi">1</span> <span class="n">c_2</span><span class="p">[</span><span class="mi">4</span><span class="p">]</span>
<span class="n">variable</span> <span class="n">diff</span> <span class="n">equal</span> <span class="n">dt</span><span class="o">*</span><span class="n">trap</span><span class="p">(</span><span class="n">f_5</span><span class="p">)</span>
<span class="n">thermo_style</span> <span class="n">custom</span> <span class="n">step</span> <span class="n">v_diff</span>
</pre></div>
</div>
<div class="admonition note">
<p class="first admonition-title">Note</p>
<p class="last">If you want the quantities calculated by this compute to be
continuous when running from a <a class="reference internal" href="read_restart.html"><span class="doc">restart file</span></a>, then
you should use the same ID for this compute, as in the original run.
This is so that the fix this compute creates to store per-atom
quantities will also have the same ID, and thus be initialized
correctly with time=0 atom velocities from the restart file.</p>
</div>
<p><strong>Output info:</strong></p>
<p>This compute calculates a global vector of length 4, which can be
accessed by indices 1-4 by any command that uses global vector values
from a compute as input. See <a class="reference internal" href="Section_howto.html#howto-15"><span class="std std-ref">this section</span></a> for an overview of LAMMPS output
options.</p>
<p>The vector values are &#8220;intensive&#8221;. The vector values will be in
velocity^2 <a class="reference internal" href="units.html"><span class="doc">units</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_msd.html"><span class="doc">compute msd</span></a></p>
<p><strong>Default:</strong> none</p>
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