<p>This doc page is not about a LAMMPS input script command, but about
body particles, which are generalized finite-size particles.
Individual body particles can represent complex entities, such as
surface meshes of discrete points, collections of sub-particles,
deformable objects, etc. Note that other kinds of finite-size
spherical and aspherical particles are also supported by LAMMPS, such
as spheres, ellipsoids, line segments, and triangles, but they are
simpler entities that body particles. See <a class="reference internal" href="Section_howto.html#howto-14"><span class="std std-ref">Section_howto 14</span></a> for a general overview of all these
particle types.</p>
<p>Body particles are used via the <a class="reference internal" href="atom_style.html"><span class="doc">atom_style body</span></a>
command. It takes a body style as an argument. The current body
styles supported by LAMMPS are as follows. The name in the first
column is used as the <em>bstyle</em> argument for the <a class="reference internal" href="atom_style.html"><span class="doc">atom_style body</span></a> command.</p>
<p>The body style determines what attributes are stored for each body and
thus how they can be used to compute pairwise body/body or
bond/non-body (point particle) interactions. More details of each
style are described below.</p>
<div class="admonition note">
<p class="first admonition-title">Note</p>
<p class="last">The rounded/polygon style listed in the table above and
described below has not yet been relesed in LAMMPS. It will be soon.</p>
</div>
<p>We hope to add more styles in the future. See <a class="reference internal" href="Section_modify.html#mod-12"><span class="std std-ref">Section_modify 12</span></a> for details on how to add a new body
style to the code.</p>
<hr class="docutils" />
<p><strong>When to use body particles:</strong></p>
<p>You should not use body particles to model a rigid body made of
simpler particles (e.g. point, sphere, ellipsoid, line segment,
triangular particles), if the interaction between pairs of rigid
bodies is just the summation of pairwise interactions between the
simpler particles. LAMMPS already supports this kind of model via the
<a class="reference internal" href="fix_rigid.html"><span class="doc">fix rigid</span></a> command. Any of the numerous pair styles
that compute interactions between simpler particles can be used. The
<a class="reference internal" href="fix_rigid.html"><span class="doc">fix rigid</span></a> command time integrates the motion of the
rigid bodies. All of the standard LAMMPS commands for thermostatting,
adding constraints, performing output, etc will operate as expected on
the simple particles.</p>
<p>By contrast, when body particles are used, LAMMPS treats an entire
body as a single particle for purposes of computing pairwise
interactions, building neighbor lists, migrating particles between
processors, outputting particles to a dump file, etc. This means that
interactions between pairs of bodies or between a body and non-body
(point) particle need to be encoded in an appropriate pair style. If
such a pair style were to mimic the <a class="reference internal" href="fix_rigid.html"><span class="doc">fix rigid</span></a> model,
it would need to loop over the entire collection of interactions
between pairs of simple particles within the two bodies, each time a
single body/body interaction was computed.</p>
<p>Thus it only makes sense to use body particles and develop such a pair
style, when particle/particle interactions are more complex than what
the <a class="reference internal" href="fix_rigid.html"><span class="doc">fix rigid</span></a> command can already calculate. For
example, if particles have one or more of the following attributes:</p>
<ul class="simple">
<li>represented by a surface mesh</li>
<li>represented by a collection of geometric entities (e.g. planes + spheres)</li>
<li>deformable</li>
<li>internal stress that induces fragmentation</li>
</ul>
<p>then the interaction between pairs of particles is likely to be more
complex than the summation of simple sub-particle interactions. An
example is contact or frictional forces between particles with planar
sufaces that inter-penetrate.</p>
<p>These are additional LAMMPS commands that can be used with body
<span class="n">Nmin</span> <span class="o">=</span> <span class="n">minimum</span> <span class="c1"># of sub-particles in any body in the system</span>
<span class="n">Nmax</span> <span class="o">=</span> <span class="n">maximum</span> <span class="c1"># of sub-particles in any body in the system</span>
</pre></div>
</div>
<p>The Nmin and Nmax arguments are used to bound the size of data
structures used internally by each particle.</p>
<p>When the <a class="reference internal" href="read_data.html"><span class="doc">read_data</span></a> command reads a data file for this
body style, the following information must be provided for each entry
in the <em>Bodies</em> section of the data file:</p>
<p>These values are the current position of the vertex within the
simulation domain, not a displacement from the center-of-mass (COM) of
the body particle itself. These values are calculated using the
current COM and orientation of the body particle.</p>
<p>For images created by the <a class="reference internal" href="dump_image.html"><span class="doc">dump image</span></a> command, if the
<em>body</em> keyword is set, then each body particle is drawn as a convex
polygon consisting of N line segments. Note that the line segments
are drawn between the N vertices, which does not correspond exactly to
the physical extent of the body (because the <a class="reference external" href="pair_body_rounded_polygon.cpp">pair_style rounded/polygon</a> defines finite-size
spheres at those point and the line segments between the spheres are
tangent to the spheres). The drawn diameter of each line segment is
determined by the <em>bflag1</em> parameter for the <em>body</em> keyword. The
<em>bflag2</em> argument is ignored.</p>
<hr class="docutils" />
<p id="fraige"><strong>(Fraige)</strong> F. Y. Fraige, P. A. Langston, A. J. Matchett, J. Dodds,
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