forked from lijiext/lammps
125 lines
4.7 KiB
HTML
125 lines
4.7 KiB
HTML
<HTML>
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<CENTER><A HREF = "http://lammps.sandia.gov">LAMMPS WWW Site</A> - <A HREF = "Manual.html">LAMMPS Documentation</A> - <A HREF = "Section_commands.html#comm">LAMMPS Commands</A>
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</CENTER>
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<HR>
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<H3>pair_style body command
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</H3>
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<P><B>Syntax:</B>
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</P>
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<PRE>pair_style body cutoff
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</PRE>
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<P>cutoff = global cutoff for interactions (distance units)
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</P>
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<P><B>Examples:</B>
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</P>
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<PRE>pair_style body 3.0
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pair_coeff * * 1.0 1.0
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pair_coeff 1 1 1.0 1.5 2.5
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</PRE>
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<P><B>Description:</B>
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</P>
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<P>Style <I>body</I> is for use with body particles and calculates pairwise
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body/body interactions as well as interactions between body and
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point-particles. See <A HREF = "Section_howto.html#howto_14">Section_howto 14</A>
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of the manual and the <A HREF = "body.html">body</A> doc page for more details on
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using body particles.
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</P>
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<P>This pair style is designed for use with the "nparticle" body style,
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which is specified as an argument to the "atom-style body" command.
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See the <A HREF = "body.html">body</A> doc page for more details about the body
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styles LAMMPS supports. The "nparticle" style treats a body particle
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as a rigid body composed of N sub-particles.
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</P>
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<P>The coordinates of a body particle are its center-of-mass (COM). If
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the COMs of a pair of body particles are within the cutoff (global or
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type-specific, as specified above), then all interactions between
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pairs of sub-particles in the two body particles are computed.
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E.g. if the first body particle has 3 sub-particles, and the second
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has 10, then 30 interactions are computed and summed to yield the
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total force and torque on each body particle.
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</P>
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<P>IMPORTANT NOTE: In the example just described, all 30 interactions are
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computed even if the distance between a particular pair of
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sub-particles is greater than the cutoff. Likewise, no interaction
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between two body particles is computed if the two COMs are further
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apart than the cutoff, even if the distance between some pairs of
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their sub-particles is within the cutoff. Thus care should be used in
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defining the cutoff distances for body particles, depending on their
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shape and size.
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</P>
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<P>Similar rules apply for a body particle interacting with a point
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particle. The distance between the two particles is calculated using
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the COM of the body particle and the position of the point particle.
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If the distance is within the cutoff and the body particle has N
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sub-particles, then N interactions with the point particle are
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computed and summed. If the distance is not within the cutoff, no
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interactions between the body and point particle are computed.
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</P>
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<P>The interaction between two sub-particles, or a sub-particle and point
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particle, or betwee two point particles is computed as a Lennard-Jones
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interaction, using the standard formula
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</P>
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<CENTER><IMG SRC = "Eqs/pair_lj.jpg">
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</CENTER>
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<P>where Rc is the cutoff. As explained above, an interaction involving
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one or two body sub-particles may be computed even for r > Rc.
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</P>
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<P>For style <I>body</I>, the following coefficients must be defined for each
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pair of atoms types via the <A HREF = "pair_coeff.html">pair_coeff</A> command as in
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the examples above, or in the data file or restart files read by the
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<A HREF = "read_data.html">read_data</A> or <A HREF = "read_restart.html">read_restart</A>
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commands:
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</P>
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<UL><LI>epsilon (energy units)
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<LI>sigma (distance units)
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<LI>cutoff (distance units)
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</UL>
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<P>The last coefficient is optional. If not specified, the global cutoff
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is used.
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</P>
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<HR>
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<P><B>Mixing, shift, table, tail correction, restart, rRESPA info</B>:
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</P>
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<P>For atom type pairs I,J and I != J, the epsilon and sigma coefficients
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and cutoff distance for all of this pair style can be mixed. The
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default mix value is <I>geometric</I>. See the "pair_modify" command for
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details.
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</P>
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<P>This pair style does not support the <A HREF = "pair_modify.html">pair_modify</A>
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shift, table, and tail options.
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</P>
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<P>This pair style does not write its information to <A HREF = "restart.html">binary restart
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files</A>.
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</P>
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<P>This pair style can only be used via the <I>pair</I> keyword of the
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<A HREF = "run_style.html">run_style respa</A> command. It does not support the
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<I>inner</I>, <I>middle</I>, <I>outer</I> keywords.
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</P>
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<HR>
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<P><B>Restrictions:</B>
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</P>
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<P>This style is part of the BODY package. It is only enabled if LAMMPS
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was built with that package. See the <A HREF = "Section_start.html#2_3">Making
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LAMMPS</A> section for more info.
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</P>
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<P>Defining particles to be bodies so they participate in body/body or
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body/particle interactions requires the use of the <A HREF = "atom_style.html">atom_style
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body</A> command.
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</P>
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<P><B>Related commands:</B>
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</P>
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<P><A HREF = "pair_coeff.html">pair_coeff</A>, <A HREF = "fix_rigid.html">fix_rigid</A>
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</P>
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<P><B>Default:</B> none
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</P>
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</HTML>
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