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<li>pair_style dsmc command</li>
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<div class="section" id="pair-style-dsmc-command">
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<span id="index-0"></span><h1>pair_style dsmc command<a class="headerlink" href="#pair-style-dsmc-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>pair_style dsmc max_cell_size seed weighting Tref Nrecompute Nsample
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</pre></div>
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</div>
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<ul class="simple">
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<li>max_cell_size = global maximum cell size for DSMC interactions (distance units)</li>
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<li>seed = random # seed (positive integer)</li>
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<li>weighting = macroparticle weighting</li>
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<li>Tref = reference temperature (temperature units)</li>
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<li>Nrecompute = recompute v*sigma_max every this many timesteps (timesteps)</li>
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<li>Nsample = sample this many times in recomputing v*sigma_max</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>pair_style dsmc 2.5 34387 10 1.0 100 20
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pair_coeff * * 1.0
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pair_coeff 1 1 1.0
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</pre></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>Style <em>dsmc</em> computes collisions between pairs of particles for a
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direct simulation Monte Carlo (DSMC) model following the exposition in
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<a class="reference internal" href="#bird"><span>(Bird)</span></a>. Each collision resets the velocities of the two
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particles involved. The number of pairwise collisions for each pair
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or particle types and the length scale within which they occur are
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determined by the parameters of the pair_style and pair_coeff
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commands.</p>
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<p>Stochastic collisions are performed using the variable hard sphere
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(VHS) approach, with the user-defined <em>max_cell_size</em> value used as
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the maximum DSMC cell size, and reference cross-sections for
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collisions given using the pair_coeff command.</p>
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<p>There is no pairwise energy or virial contributions associated with
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this pair style.</p>
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<p>The following coefficient must be defined for each pair of atoms types
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via the <a class="reference internal" href="pair_coeff.html"><em>pair_coeff</em></a> command as in the examples above,
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or in the data file or restart files read by the
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<a class="reference internal" href="read_data.html"><em>read_data</em></a> or <a class="reference internal" href="read_restart.html"><em>read_restart</em></a>
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commands:</p>
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<ul class="simple">
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<li>sigma (area units, i.e. distance-squared)</li>
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</ul>
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<p>The global DSMC <em>max_cell_size</em> determines the maximum cell length
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used in the DSMC calculation. A structured mesh is overlayed on the
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simulation box such that an integer number of cells are created in
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each direction for each processor’s sub-domain. Cell lengths are
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adjusted up to the user-specified maximum cell size.</p>
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<hr class="docutils" />
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<p>To perform a DSMC simulation with LAMMPS, several additional options
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should be set in your input script, though LAMMPS does not check for
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these settings.</p>
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<p>Since this pair style does not compute particle forces, you should use
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the “fix nve/noforce” time integration fix for the DSMC particles,
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e.g.</p>
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<div class="highlight-python"><div class="highlight"><pre>fix 1 all nve/noforce
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</pre></div>
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</div>
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<p>This pair style assumes that all particles will communicated to
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neighboring processors every timestep as they move. This makes it
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possible to perform all collisions between pairs of particles that are
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on the same processor. To ensure this occurs, you should use
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these commands:</p>
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<div class="highlight-python"><div class="highlight"><pre>neighbor 0.0 bin
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neigh_modify every 1 delay 0 check no
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atom_modify sort 0 0.0
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communicate single cutoff 0.0
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</pre></div>
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</div>
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<p>These commands ensure that LAMMPS communicates particles to
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neighboring processors every timestep and that no ghost atoms are
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created. The output statistics for a simulation run should indicate
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there are no ghost particles or neighbors.</p>
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<p>In order to get correct DSMC collision statistics, users should
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specify a Gaussian velocity distribution when populating the
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simulation domain. Note that the default velocity distribution is
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uniform, which will not give good DSMC collision rates. Specify
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“dist gaussian” when using the <a class="reference internal" href="velocity.html"><em>velocity</em></a> command
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as in the following:</p>
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<div class="highlight-python"><div class="highlight"><pre>velocity all create 594.6 87287 loop geom dist gaussian
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</pre></div>
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</div>
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<hr class="docutils" />
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<p><strong>Mixing, shift, table, tail correction, restart, rRESPA info</strong>:</p>
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<p>This pair style does not support mixing. Thus, coefficients for all
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I,J pairs must be specified explicitly.</p>
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<p>This pair style does not support the <a class="reference internal" href="pair_modify.html"><em>pair_modify</em></a>
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shift option for the energy of the pair interaction.</p>
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<p>The <a class="reference internal" href="pair_modify.html"><em>pair_modify</em></a> table option is not relevant
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for this pair style.</p>
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<p>This pair style does not support the <a class="reference internal" href="pair_modify.html"><em>pair_modify</em></a>
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tail option for adding long-range tail corrections to energy and
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pressure.</p>
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<p>This pair style writes its information to <a class="reference internal" href="restart.html"><em>binary restart files</em></a>, so pair_style and pair_coeff commands do not need
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to be specified in an input script that reads a restart file. Note
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that the user-specified random number seed is stored in the restart
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file, so when a simulation is restarted, each processor will
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re-initialize its random number generator the same way it did
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initially. This means the random forces will be random, but will not
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be the same as they would have been if the original simulation had
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continued past the restart time.</p>
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<p>This pair style can only be used via the <em>pair</em> keyword of the
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<a class="reference internal" href="run_style.html"><em>run_style respa</em></a> command. It does not support the
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<em>inner</em>, <em>middle</em>, <em>outer</em> keywords.</p>
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</div>
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<hr class="docutils" />
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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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<p>This style is part of the MC package. It is only enabled if LAMMPS
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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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</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="pair_coeff.html"><em>pair_coeff</em></a>, <a class="reference internal" href="fix_nve_noforce.html"><em>fix nve/noforce</em></a>,
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<a class="reference internal" href="neigh_modify.html"><em>neigh_modify</em></a>, <a class="reference internal" href="neighbor.html"><em>neighbor</em></a>,
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<a class="reference internal" href="comm_modify.html"><em>comm_modify</em></a></p>
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<p><strong>Default:</strong> none</p>
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<hr class="docutils" />
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<p id="bird"><strong>(Bird)</strong> G. A. Bird, “Molecular Gas Dynamics and the Direct Simulation
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of Gas Flows” (1994).</p>
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