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<li>fix lb/rigid/pc/sphere command</li>
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<div class="section" id="fix-lb-rigid-pc-sphere-command">
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<span id="index-0"></span><h1>fix lb/rigid/pc/sphere command<a class="headerlink" href="#fix-lb-rigid-pc-sphere-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 lb/rigid/pc/sphere bodystyle args keyword values ...
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</pre></div>
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</div>
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<ul>
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<li><p class="first">ID, group-ID are documented in <a class="reference internal" href="fix.html"><em>fix</em></a> command</p>
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</li>
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<li><p class="first">lb/rigid/pc/sphere = style name of this fix command</p>
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</li>
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<li><p class="first">bodystyle = <em>single</em> or <em>molecule</em> or <em>group</em></p>
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</li>
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<li><dl class="first docutils">
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<dt><em>single</em> args = none</dt>
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<dd><p class="first"><em>molecule</em> args = none
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<em>group</em> args = N groupID1 groupID2 ...</p>
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<blockquote class="last">
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<div><p>N = # of groups</p>
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</div></blockquote>
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</dd>
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</dl>
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<p>zero or more keyword/value pairs may be appended</p>
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</li>
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<li><p class="first">keyword = <em>force</em> or <em>torque</em> or <em>innerNodes</em></p>
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</li>
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</ul>
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<pre class="literal-block">
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<em>force</em> values = M xflag yflag zflag
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M = which rigid body from 1-Nbody (see asterisk form below)
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xflag,yflag,zflag = off/on if component of center-of-mass force is active
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<em>torque</em> values = M xflag yflag zflag
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M = which rigid body from 1-Nbody (see asterisk form below)
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xflag,yflag,zflag = off/on if component of center-of-mass torque is active
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<em>innerNodes</em> values = innergroup-ID
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innergroup-ID = ID of the atom group which does not experience a hydrodynamic force from the lattice-Boltzmann fluid
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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 spheres lb/rigid/pc/sphere
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fix 1 all lb/rigid/pc/sphere force 1 0 0 innerNodes ForceAtoms
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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>This fix is based on the <a class="reference internal" href="fix_rigid.html"><em>fix rigid</em></a> command, and was
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created to be used in place of that fix, to integrate the equations of
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motion of spherical rigid bodies when a lattice-Boltzmann fluid is
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present with a user-specified value of the force-coupling constant.
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The fix uses the integration algorithm described in <a class="reference internal" href="fix_lb_viscous.html#mackay"><span>Mackay et al.</span></a> to update the positions, velocities, and orientations of
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a set of spherical rigid bodies experiencing velocity dependent
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hydrodynamic forces. The spherical bodies are assumed to rotate as
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solid, uniform density spheres, with moments of inertia calculated
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using the combined sum of the masses of all the constituent particles
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(which are assumed to be point particles).</p>
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<hr class="docutils" />
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<p>By default, all of the atoms that this fix acts on experience a
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hydrodynamic force due to the presence of the lattice-Boltzmann fluid.
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However, the <em>innerNodes</em> keyword allows the user to specify atoms
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belonging to a rigid object which do not interact with the
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lattice-Boltzmann fluid (i.e. these atoms do not feel a hydrodynamic
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force from the lattice-Boltzmann fluid). This can be used to
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distinguish between atoms on the surface of a non-porous object, and
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those on the inside.</p>
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<p>This feature can be used, for example, when implementing a hard sphere
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interaction between two spherical objects. Instead of interactions
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occurring between the particles on the surfaces of the two spheres, it
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is desirable simply to place an atom at the center of each sphere,
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which does not contribute to the hydrodynamic force, and have these
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central atoms interact with one another.</p>
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<hr class="docutils" />
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<p>Apart from the features described above, this fix is very similar to
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the rigid fix (although it includes fewer optional arguments, and
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assumes the constituent atoms are point particles); see
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<a class="reference internal" href="fix_rigid.html"><em>fix rigid</em></a> for a complete documentation.</p>
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</div>
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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 the <em>rigid</em> and <em>rigid/nve</em> fixes are written to
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<a class="reference internal" href="restart.html"><em>binary restart files</em></a>.</p>
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<p>Similar to the <a class="reference internal" href="fix_rigid.html"><em>fix rigid</em></a> command: &quot; The rigid
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fix computes a global scalar which can be accessed by various <a class="reference internal" href="Section_howto.html#howto-15"><span>output commands</span></a>. The scalar value calculated by
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these fixes is “intensive”. The scalar is the current temperature of
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the collection of rigid bodies. This is averaged over all rigid
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bodies and their translational and rotational degrees of freedom. The
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translational energy of a rigid body is 1/2 m v^2, where m = total
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mass of the body and v = the velocity of its center of mass. The
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rotational energy of a rigid body is 1/2 I w^2, where I = the moment
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of inertia tensor of the body and w = its angular velocity. Degrees
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of freedom constrained by the <em>force</em> and <em>torque</em> keywords are
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removed from this calculation.&quot;</p>
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<p>&quot;All of these fixes compute a global array of values which can be
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accessed by various <a class="reference internal" href="Section_howto.html#howto-15"><span>output commands</span></a>.
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The number of rows in the array is equal to the number of rigid
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bodies. The number of columns is 15. Thus for each rigid body, 15
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values are stored: the xyz coords of the center of mass (COM), the xyz
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components of the COM velocity, the xyz components of the force acting
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on the COM, the xyz components of the torque acting on the COM, and
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the xyz image flags of the COM, which have the same meaning as image
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flags for atom positions (see the “dump” command). The force and
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torque values in the array are not affected by the <em>force</em> and
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<em>torque</em> keywords in the fix rigid command; they reflect values before
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any changes are made by those keywords.</p>
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<p>The ordering of the rigid bodies (by row in the array) is as follows.
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For the <em>single</em> keyword there is just one rigid body. For the
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<em>molecule</em> keyword, the bodies are ordered by ascending molecule ID.
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For the <em>group</em> keyword, the list of group IDs determines the ordering
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of bodies.</p>
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<p>The array values calculated by these fixes are “intensive”, meaning
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they are independent of the number of atoms in the simulation.</p>
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<p>No parameter of these fixes can be used with the <em>start/stop</em> keywords
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of the <a class="reference internal" href="run.html"><em>run</em></a> command. These fixes are not invoked during
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<a class="reference internal" href="minimize.html"><em>energy minimization</em></a>. &quot;</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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<p>This fix is part of the USER-LB 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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<p>Can only be used if a lattice-Boltzmann fluid has been created via the
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<a class="reference internal" href="fix_lb_fluid.html"><em>fix lb/fluid</em></a> command, and must come after this
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command. Should only be used if the force coupling constant used in
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<a class="reference internal" href="fix_lb_fluid.html"><em>fix lb/fluid</em></a> has been set by the user; this
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integration fix cannot be used if the force coupling constant is set
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by default.</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="fix_lb_fluid.html"><em>fix lb/fluid</em></a>, <a class="reference internal" href="fix_lb_pc.html"><em>fix lb/pc</em></a></p>
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</div>
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<div class="section" id="default">
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<h2>Default<a class="headerlink" href="#default" title="Permalink to this headline">¶</a></h2>
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<p>The defaults are force * on on on, and torque * on on on.</p>
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<hr class="docutils" />
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<p id="mackay"><strong>(Mackay et al.)</strong> Mackay, F. E., Ollila, S.T.T., and Denniston, C., Hydrodynamic Forces Implemented into LAMMPS through a lattice-Boltzmann fluid, Computer Physics Communications 184 (2013) 2021-2031.</p>
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