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<li class="toctree-l1"><a class="reference internal" href="Section_intro.html">1. Introduction</a></li>
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<li>compute vacf command</li>
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<div class="section" id="compute-vacf-command">
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<span id="index-0"></span><h1>compute vacf command<a class="headerlink" href="#compute-vacf-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>compute ID group-ID vacf
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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="compute.html"><em>compute</em></a> command</li>
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<li>vacf = style name of this compute command</li>
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</ul>
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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>compute 1 all vacf
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compute 1 upper vacf
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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>Define a computation that calculates the velocity auto-correlation
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function (VACF), averaged over a group of atoms. Each atom’s
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contribution to the VACF is its current velocity vector dotted into
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its initial velocity vector at the time the compute was specified.</p>
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<p>A vector of four quantites is calculated by this compute. The first 3
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elements of the vector are vx * vx0 (and similarly for the y and z
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components), summed and averaged over atoms in the group. Vx is the
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current x-component of velocity for the atom, vx0 is the initial
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x-component of velocity for the atom. The 4th element of the vector
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is the total VACF, i.e. (vx*vx0 + vy*vy0 + vz*vz0), summed and
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averaged over atoms in the group.</p>
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<p>The integral of the VACF versus time is proportional to the diffusion
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coefficient of the diffusing atoms. This can be computed in the
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following manner, using the <a class="reference internal" href="variable.html"><em>variable trap()</em></a> function:</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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<div class="admonition note">
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<p class="first admonition-title">Note</p>
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<p class="last">If you want the quantities calculated by this compute to be
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continuous when running from a <a class="reference internal" href="read_restart.html"><em>restart file</em></a>, then
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you should use the same ID for this compute, as in the original run.
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This is so that the fix this compute creates to store per-atom
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quantities will also have the same ID, and thus be initialized
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correctly with time=0 atom velocities from the restart file.</p>
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</div>
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<p><strong>Output info:</strong></p>
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<p>This compute calculates a global vector of length 4, which can be
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accessed by indices 1-4 by any command that uses global vector values
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from a compute as input. See <a class="reference internal" href="Section_howto.html#howto-15"><span>this section</span></a> for an overview of LAMMPS output
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options.</p>
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<p>The vector values are “intensive”. The vector values will be in
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velocity^2 <a class="reference internal" href="units.html"><em>units</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_msd.html"><em>compute msd</em></a></p>
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<p><strong>Default:</strong> none</p>
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