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<li class="breadcrumb-item active"><span class="section-number">8.5.2. </span>Granular models</li>
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<p><span class="math notranslate nohighlight">\(\renewcommand{\AA}{\text{Å}}\)</span></p>
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<section id="granular-models">
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<h1><span class="section-number">8.5.2. </span>Granular models<a class="headerlink" href="#granular-models" title="Link to this heading"></a></h1>
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<p>Granular system are composed of spherical particles with a diameter,
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as opposed to point particles. This means they have an angular
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velocity and torque can be imparted to them to cause them to rotate.</p>
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<p>To run a simulation of a granular model, you will want to use
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the following commands:</p>
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<ul class="simple">
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<li><p><a class="reference internal" href="atom_style.html"><span class="doc">atom_style sphere</span></a></p></li>
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<li><p><a class="reference internal" href="fix_nve_sphere.html"><span class="doc">fix nve/sphere</span></a></p></li>
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<li><p><a class="reference internal" href="fix_gravity.html"><span class="doc">fix gravity</span></a></p></li>
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</ul>
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<p>This compute</p>
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<ul class="simple">
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<li><p><a class="reference internal" href="compute_erotate_sphere.html"><span class="doc">compute erotate/sphere</span></a></p></li>
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</ul>
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<p>calculates rotational kinetic energy which can be <a class="reference internal" href="Howto_output.html"><span class="doc">output with thermodynamic info</span></a>.
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The compute</p>
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<ul class="simple">
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<li><p><a class="reference internal" href="compute_fabric.html"><span class="doc">compute fabric</span></a></p></li>
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</ul>
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<p>calculates various versions of the fabric tensor for granular and non-granular pair styles.</p>
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<p>Use one of these 4 pair potentials, which compute forces and torques
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between interacting pairs of particles:</p>
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<ul class="simple">
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<li><p><a class="reference internal" href="pair_gran.html"><span class="doc">pair_style gran/history</span></a></p></li>
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<li><p><a class="reference internal" href="pair_gran.html"><span class="doc">pair_style gran/no_history</span></a></p></li>
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<li><p><a class="reference internal" href="pair_gran.html"><span class="doc">pair_style gran/hertzian</span></a></p></li>
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<li><p><a class="reference internal" href="pair_granular.html"><span class="doc">pair_style granular</span></a></p></li>
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</ul>
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<p>These commands implement fix options specific to granular systems:</p>
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<ul class="simple">
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<li><p><a class="reference internal" href="fix_freeze.html"><span class="doc">fix freeze</span></a></p></li>
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<li><p><a class="reference internal" href="fix_pour.html"><span class="doc">fix pour</span></a></p></li>
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<li><p><a class="reference internal" href="fix_viscous.html"><span class="doc">fix viscous</span></a></p></li>
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<li><p><a class="reference internal" href="fix_wall_gran.html"><span class="doc">fix wall/gran</span></a></p></li>
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<li><p><a class="reference internal" href="fix_wall_gran_region.html"><span class="doc">fix wall/gran/region</span></a></p></li>
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</ul>
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<p>The fix style <em>freeze</em> zeroes both the force and torque of frozen
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atoms, and should be used for granular system instead of the fix style
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<em>setforce</em>.</p>
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<p>To model heat conduction, one must add the temperature and heatflow
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atom variables with:</p>
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<ul class="simple">
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<li><p><a class="reference internal" href="fix_property_atom.html"><span class="doc">fix property/atom</span></a></p></li>
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</ul>
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<p>a temperature integration fix</p>
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<ul class="simple">
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<li><p><a class="reference internal" href="fix_heat_flow.html"><span class="doc">fix heat/flow</span></a></p></li>
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</ul>
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<p>and a heat conduction option defined in both</p>
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<ul class="simple">
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<li><p><a class="reference internal" href="pair_granular.html"><span class="doc">pair_style granular</span></a></p></li>
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<li><p><a class="reference internal" href="fix_wall_gran.html"><span class="doc">fix wall/gran</span></a></p></li>
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</ul>
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<p>For computational efficiency, you can eliminate needless pairwise
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computations between frozen atoms by using this command:</p>
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<ul class="simple">
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<li><p><a class="reference internal" href="neigh_modify.html"><span class="doc">neigh_modify</span></a> exclude</p></li>
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</ul>
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<div class="admonition note">
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<p class="admonition-title">Note</p>
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<p>By default, for 2d systems, granular particles are still modeled
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as 3d spheres, not 2d discs (circles), meaning their moment of inertia
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will be the same as in 3d. If you wish to model granular particles in
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2d as 2d discs, see the note on this topic on the <a class="reference internal" href="Howto_2d.html"><span class="doc">Howto 2d</span></a>
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doc page, where 2d simulations are discussed.</p>
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</div>
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<p>To add custom granular contact models, see the
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<a class="reference internal" href="Modify_gran_sub_mod.html"><span class="doc">modifying granular sub-models page</span></a>.</p>
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</section>
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