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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>
</CENTER>
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<H3>communicate command
</H3>
<P><B>Syntax:</B>
</P>
<PRE>communicate style keyword value ...
</PRE>
<UL><LI>style = <I>single</I> or <I>multi</I>
<LI>zero or more keyword/value pairs may be appended
<LI>keyword = <I>group</I> or <I>cutoff</I>
<PRE> <I>group</I> value = group-ID = only communicate atoms in the group
<I>cutoff</I> value = Rcut (distance units) = communicate atoms from this far away
</PRE>
</UL>
<P><B>Examples:</B>
</P>
<PRE>communicate multi
communicate multi group solvent
communicate single cutoff 5.0
</PRE>
<P><B>Description:</B>
</P>
<P>This command sets the style of inter-processor communication that
occurs each timestep as atom coordinates and other properties are
exchanged between neighboring processors and stored as properties of
ghost atoms.
</P>
<P>The default style is <I>single</I> which means each processor acquires
information for ghost atoms that are within a single distance from its
sub-domain. The distance is the maximum of the neighbor cutoff for
all atom type pairs.
</P>
<P>For many systems this is an efficient algorithm, but for systems with
widely varying cutoffs for different type pairs, the <I>multi</I> style can
be faster. In this case, each atom type is assigned its own distance
cutoff for communication purposes, and fewer atoms will be
communicated. See the <A HREF = "neighbor.html">neighbor multi</A> command for a
neighbor list construction option that may also be beneficial for
simulations of this kind.
</P>
<P>The <I>group</I> option will limit communication to atoms in the specified
group. This can be useful for models where no ghost atoms are needed
for some kinds of particles. All atoms (not just those in the
specified group) will still migrate to new processors as they move.
The group specified with this option must also be specified via the
<A HREF = "atom_modify.html">atom_modify first</A> command.
</P>
<P>The <I>cutoff</I> option allows you to set a ghost cutoff distance, which
is the distance from the borders of a processor's sub-domain at which
ghost atoms are acquired from other processors. By default the ghost
cutoff = neighbor cutoff = pairwise force cutoff + neighbor skin. See
the <A HREF = "neighbor.html">neighbor</A> command for more information about the
skin distance. If the specified Rcut is greater than the neighbor
cutoff, then extra ghost atoms will be acquired. If it is smaller,
the ghost cutoff is set to the neighbor cutoff.
</P>
<P>These are simulation scenarios in which it may be useful to set a
ghost cutoff > neighbor cutoff:
</P>
<UL><LI>a single polymer chain with bond interactions, but no pairwise interactions
<LI>bonded interactions (e.g. dihedrals) extend further than the pairwise cutoff
<LI>ghost atoms beyond the pairwise cutoff are needed for some computation
</UL>
<P>In the first scenario, a pairwise potential may not be defined. Thus
the pairwise neighbor cutoff will be 0.0. But ghost atoms are still
needed for computing bond, angle, etc interactions between atoms on
different processors. The appropriate ghost cutoff depends on the
<A HREF = "newton.html">newton bond</A> setting. For newton bond <I>off</I>, the
distance needs to be the furthest distance between any two atoms in
the bond, angle, etc. E.g. the distance between 1-4 atoms in a
dihedral. For newton bond <I>on</I>, the distance between the central atom
in the bond, angle, etc and any other atom is sufficient. E.g. the
distance between 2-4 atoms in a dihedral.
</P>
<P>In the second scenario, a pairwise potential is defined, but its
neighbor cutoff is not sufficiently long enough to enable bond, angle,
etc terms to be computed. As in the previous scenario, an appropriate
ghost cutoff should be set.
</P>
<P>In the last scenario, a <A HREF = "fix.html">fix</A> or <A HREF = "compute.html">compute</A> or
<A HREF = "pair_style.html">pairwise potential</A> needs to calculate with ghost
atoms beyond the normal pairwise cutoff for some computation it
performs (e.g. locate neighbors of ghost atoms in a multibody pair
potential). Setting the ghost cutoff appropriately can insure it will
find the needed atoms.
</P>
<P><B>Restrictions:</B> none
</P>
<P><B>Related commands:</B>
</P>
<P><A HREF = "neighbor.html">neighbor</A>
</P>
<P><B>Default:</B>
</P>
<P>The default settings are style = single, group = all, cutoff = 0.0.
The cutoff default of 0.0 means that effectively ghost cutoff =
neighbor cutoff = pairwise force cutoff + neighbor skin.
</P>
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