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