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<div class="section" id="fix-viscous-command">
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<span id="index-0"></span><h1>fix viscous command<a class="headerlink" href="#fix-viscous-command" title="Permalink to this headline">¶</a></h1>
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</div>
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<div class="section" id="fix-viscous-cuda-command">
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<h1>fix viscous/cuda command<a class="headerlink" href="#fix-viscous-cuda-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>fix ID group-ID viscous gamma keyword values ...
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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"><em>fix</em></a> command</li>
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<li>viscous = style name of this fix command</li>
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<li>gamma = damping coefficient (force/velocity units)</li>
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<li>zero or more keyword/value pairs may be appended</li>
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</ul>
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<pre class="literal-block">
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keyword = <em>scale</em>
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<em>scale</em> values = type ratio
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type = atom type (1-N)
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ratio = factor to scale the damping coefficient by
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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<a class="headerlink" href="#examples" title="Permalink to this headline">¶</a></h2>
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<div class="highlight-python"><div class="highlight"><pre>fix 1 flow viscous 0.1
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fix 1 damp viscous 0.5 scale 3 2.5
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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>Add a viscous damping force to atoms in the group that is proportional
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to the velocity of the atom. The added force can be thought of as a
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frictional interaction with implicit solvent, i.e. the no-slip Stokes
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drag on a spherical particle. In granular simulations this can be
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useful for draining the kinetic energy from the system in a controlled
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fashion. If used without additional thermostatting (to add kinetic
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energy to the system), it has the effect of slowly (or rapidly)
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freezing the system; hence it can also be used as a simple energy
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minimization technique.</p>
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<p>The damping force F is given by F = - gamma * velocity. The larger
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the coefficient, the faster the kinetic energy is reduced. If the
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optional keyword <em>scale</em> is used, gamma can scaled up or down by the
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specified factor for atoms of that type. It can be used multiple
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times to adjust gamma for several atom types.</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">You should specify gamma in force/velocity units. This is not
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the same as mass/time units, at least for some of the LAMMPS
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<a class="reference internal" href="units.html"><em>units</em></a> options like “real” or “metal” that are not
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self-consistent.</p>
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</div>
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<p>In a Brownian dynamics context, gamma = Kb T / D, where Kb =
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Boltzmann’s constant, T = temperature, and D = particle diffusion
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coefficient. D can be written as Kb T / (3 pi eta d), where eta =
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dynamic viscosity of the frictional fluid and d = diameter of
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particle. This means gamma = 3 pi eta d, and thus is proportional to
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the viscosity of the fluid and the particle diameter.</p>
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<p>In the current implementation, rather than have the user specify a
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viscosity, gamma is specified directly in force/velocity units. If
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needed, gamma can be adjusted for atoms of different sizes
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(i.e. sigma) by using the <em>scale</em> keyword.</p>
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<p>Note that Brownian dynamics models also typically include a randomized
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force term to thermostat the system at a chosen temperature. The <a class="reference internal" href="fix_langevin.html"><em>fix langevin</em></a> command does this. It has the same
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viscous damping term as fix viscous and adds a random force to each
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atom. The random force term is proportional to the sqrt of the chosen
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thermostatting temperature. Thus if you use fix langevin with a
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target T = 0, its random force term is zero, and you are essentially
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performing the same operation as fix viscous. Also note that the
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gamma of fix viscous is related to the damping parameter of <a class="reference internal" href="fix_langevin.html"><em>fix langevin</em></a>, however the former is specified in units
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of force/velocity and the latter in units of time, so that it can more
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easily be used as a thermostat.</p>
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<hr class="docutils" />
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<p>Styles with a <em>cuda</em> suffix are functionally the same as the
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corresponding style without the suffix. They have been optimized to
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run faster, depending on your available hardware, as discussed in
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<a class="reference internal" href="Section_accelerate.html"><em>Section_accelerate</em></a> of the manual. The
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accelerated styles take the same arguments and should produce the same
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results, except for round-off and precision issues.</p>
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<p>These accelerated styles are part of the USER-CUDA package. They are
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only enabled if LAMMPS was built with that package. See the <a class="reference internal" href="Section_start.html#start-3"><span>Making LAMMPS</span></a> section for more info.</p>
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<p>You can specify the accelerated styles explicitly in your input script
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by including their suffix, or you can use the <a class="reference internal" href="Section_start.html#start-7"><span>-suffix command-line switch</span></a> when you invoke LAMMPS, or you can
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use the <a class="reference internal" href="suffix.html"><em>suffix</em></a> command in your input script.</p>
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<p>See <a class="reference internal" href="Section_accelerate.html"><em>Section_accelerate</em></a> of the manual for
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more instructions on how to use the accelerated styles effectively.</p>
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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<a class="headerlink" href="#restart-fix-modify-output-run-start-stop-minimize-info" title="Permalink to this headline">¶</a></h2>
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<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
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are relevant to this fix. No global or per-atom quantities are stored
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by this fix for access by various <a class="reference internal" href="Section_howto.html#howto-15"><span>output commands</span></a>. No parameter of this fix can
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be used with the <em>start/stop</em> keywords of the <a class="reference internal" href="run.html"><em>run</em></a> command.</p>
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<p>The forces due to this fix are imposed during an energy minimization,
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invoked by the <a class="reference internal" href="minimize.html"><em>minimize</em></a> command. This fix should only
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be used with damped dynamics minimizers that allow for
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non-conservative forces. See the <a class="reference internal" href="min_style.html"><em>min_style</em></a> command
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for details.</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="fix_langevin.html"><em>fix langevin</em></a></p>
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<p><strong>Default:</strong> none</p>
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