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<div class="section" id="fix-flow-gauss-command">
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<span id="index-0"></span><h1>fix flow/gauss command</h1>
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<div class="section" id="syntax">
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<h2>Syntax</h2>
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<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">flow</span><span class="o">/</span><span class="n">gauss</span> <span class="n">xflag</span> <span class="n">yflag</span> <span class="n">zflag</span> <span class="n">keyword</span>
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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"><span class="doc">fix</span></a> command</li>
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<li>flow/gauss = style name of this fix command</li>
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<li>xflag,yflag,zflag = 0 or 1</li>
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</ul>
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<div class="highlight-default"><div class="highlight"><pre><span></span><span class="mi">0</span> <span class="o">=</span> <span class="n">do</span> <span class="ow">not</span> <span class="n">conserve</span> <span class="n">current</span> <span class="ow">in</span> <span class="n">this</span> <span class="n">dimension</span>
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<span class="mi">1</span> <span class="o">=</span> <span class="n">conserve</span> <span class="n">current</span> <span class="ow">in</span> <span class="n">this</span> <span class="n">dimension</span>
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</pre></div>
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</div>
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<ul class="simple">
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<li>zero or more keyword/value pairs may be appended</li>
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<li>keyword = <em>energy</em></li>
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</ul>
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<pre class="literal-block">
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<em>energy</em> value = no or yes
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no = do not compute work done by this fix
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yes = compute work done by this fix
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</pre>
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</div>
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<div class="section" id="examples">
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<h2>Examples</h2>
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<div class="highlight-default"><div class="highlight"><pre><span></span><span class="n">fix</span> <span class="n">GD</span> <span class="n">fluid</span> <span class="n">flow</span><span class="o">/</span><span class="n">gauss</span> <span class="mi">1</span> <span class="mi">0</span> <span class="mi">0</span>
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<span class="n">fix</span> <span class="n">GD</span> <span class="n">fluid</span> <span class="n">flow</span><span class="o">/</span><span class="n">gauss</span> <span class="mi">1</span> <span class="mi">1</span> <span class="mi">1</span> <span class="n">energy</span> <span class="n">yes</span>
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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</h2>
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<p>This fix implements the Gaussian dynamics (GD) method to simulate a system at
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constant mass flux <a class="reference internal" href="#strong"><span class="std std-ref">(Strong)</span></a>. GD is a nonequilibrium molecular
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dynamics simulation method that can be used to study fluid flows through
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pores, pipes, and channels. In its original implementation GD was used to
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compute the pressure required to achieve a fixed mass flux through an opening.
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The flux can be conserved in any combination of the directions, x, y, or z,
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using xflag,yflag,zflag. This fix does not initialize a net flux through
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a system, it only conserves the center-of-mass momentum that is present
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when the fix is declared in the input script. Use the <a class="reference internal" href="velocity.html"><span class="doc">velocity</span></a>
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command to generate an initial center-of-mass momentum.</p>
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<p>GD applies an external fluctuating gravitational field that acts as a driving
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force to keep the system away from equilibrium. To maintain steady state, a
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profile-unbiased thermostat must be implemented to dissipate the heat that is
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added by the driving force. <a class="reference internal" href="compute_temp_profile.html"><span class="doc">Compute temp/profile</span></a>
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can be used to implement a profile-unbiased thermostat.</p>
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<p>A common use of this fix is to compute a pressure drop across a pipe, pore, or
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membrane. The pressure profile can be computed in LAMMPS with <a class="reference internal" href="compute_stress_atom.html"><span class="doc">compute stress/atom</span></a> and <a class="reference internal" href="fix_ave_chunk.html"><span class="doc">fix ave/chunk</span></a>,
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or with the hardy method in <a class="reference internal" href="fix_atc.html"><span class="doc">fix atc</span></a>. Note that the simple
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<a class="reference internal" href="compute_stress_atom.html"><span class="doc">compute stress/atom</span></a> method is only accurate away
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from inhomogeneities in the fluid, such as fixed wall atoms. Further, the
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computed pressure profile must be corrected for the acceleration applied by
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GD before computing a pressure drop or comparing it to other methods, such as
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the pump method <a class="reference internal" href="#zhu"><span class="std std-ref">(Zhu)</span></a>. The pressure correction is discussed and
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described in <a class="reference internal" href="#strong"><span class="std std-ref">(Strong)</span></a>.</p>
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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">For a complete example including the considerations discussed above, see
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the examples/USER/flow_gauss directory.</p>
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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">Only the flux of the atoms in group-ID will be conserved. If the
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velocities of the group-ID atoms are coupled to the velocities of other atoms
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in the simulation, the flux will not be conserved. For example, in a
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simulation with fluid atoms and harmonically constrained wall atoms, if a
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single thermostat is applied to group <em>all</em>, the fluid atom velocities will be
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coupled to the wall atom velocities, and the flux will not be conserved. This
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issue can be avoided by thermostatting the fluid and wall groups separately.</p>
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</div>
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<dl class="docutils">
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<dt>Adding an acceleration to atoms does work on the system. This added energy</dt>
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<dd>can be optionally subtracted from the potential energy for the thermodynamic</dd>
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</dl>
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<p>output (see below) to check that the timestep is small enough to conserve
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energy. Since the applied acceleration is fluctuating in time, the work cannot
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be computed from a potential. As a result, computing the work is slightly more
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computationally expensive than usual, so it is not performed by default. To
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invoke the work calculation, use the <em>energy</em> keyword. The
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<a class="reference internal" href="fix_modify.html"><span class="doc">fix_modify</span></a> <em>energy</em> option also invokes the work
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calculation, and overrides an <em>energy no</em> setting here. If neither <em>energy yes</em>
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or <em>fix_modify energy yes</em> are set, the global scalar computed by the fix
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will return zero.</p>
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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">In order to check energy conservation, any other fixes that do work on
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the system must have <em>fix_modify energy yes</em> set as well. This includes
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thermostat fixes and any constraints that hold the positions of wall atoms
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fixed, such as <a class="reference internal" href="fix_spring_self.html"><span class="doc">fix spring/self</span></a>.</p>
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</div>
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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</h2>
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<p>This fix is part of the USER-MISC package. It is only enabled if
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LAMMPS was built with that package. See the <a class="reference internal" href="Section_start.html#start-3"><span class="std std-ref">Making LAMMPS</span></a> section for more info.</p>
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<p>No information about this fix is written to <a class="reference internal" href="restart.html"><span class="doc">binary restart files</span></a>.</p>
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<p>The <a class="reference internal" href="fix_modify.html"><span class="doc">fix_modify</span></a> <em>energy</em> option is supported by this
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fix to subtract the work done from the
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system’s potential energy as part of <a class="reference internal" href="thermo_style.html"><span class="doc">thermodynamic output</span></a>.</p>
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<p>This fix computes a global scalar and a global 3-vector of forces,
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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>. The scalar is the negative of the
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work done on the system, see above discussion. The vector is the total force
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that this fix applied to the group of atoms on the current timestep.
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The scalar and vector values calculated by this fix are “extensive”.</p>
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<p>No parameter of this fix can be used with the <em>start/stop</em> keywords of
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the <a class="reference internal" href="run.html"><span class="doc">run</span></a> command.</p>
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</div>
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<div class="section" id="restrictions">
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<h2>Restrictions</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</h2>
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<p><a class="reference internal" href="fix_addforce.html"><span class="doc">fix addforce</span></a>, <a class="reference internal" href="compute_temp_profile.html"><span class="doc">compute temp/profile</span></a>, <a class="reference internal" href="velocity.html"><span class="doc">velocity</span></a></p>
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</div>
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<div class="section" id="default">
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<h2>Default</h2>
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<p>The option default for the <em>energy</em> keyword is energy = no.</p>
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<hr class="docutils" />
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<p id="strong"><strong>(Strong)</strong> Strong and Eaves, J. Phys. Chem. Lett. 7, 1907 (2016).</p>
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<p id="evans"><strong>(Evans)</strong> Evans and Morriss, Phys. Rev. Lett. 56, 2172 (1986).</p>
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<p id="zhu"><strong>(Zhu)</strong> Zhu, Tajkhorshid, and Schulten, Biophys. J. 83, 154 (2002).</p>
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<hr class="docutils" />
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