2009-12-22 07:01:58 +08:00
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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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2009-12-22 08:22:16 +08:00
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<H3>compute pair/local command
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2009-12-22 07:01:58 +08:00
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</H3>
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<P><B>Syntax:</B>
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</P>
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2009-12-22 08:22:16 +08:00
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<PRE>compute ID group-ID pair/local input1 input2 ...
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2009-12-22 07:01:58 +08:00
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</PRE>
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<UL><LI>ID, group-ID are documented in <A HREF = "compute.html">compute</A> command
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2009-12-22 08:22:16 +08:00
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<LI>pair/local = style name of this compute command
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2009-12-22 07:01:58 +08:00
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<LI>zero or more keywords may be appended
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2012-02-13 23:49:36 +08:00
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<LI>keyword = <I>dist</I> or <I>eng</I> or <I>force</I> or <I>fx</I> or <I>fy</I> or <I>fz</I> or <I>pN</I>
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2009-12-22 07:01:58 +08:00
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2011-12-01 08:19:11 +08:00
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<PRE> <I>dist</I> = pairwise distance
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<I>eng</I> = pairwise energy
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<I>force</I> = pairwise force
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<I>fx</I>,<I>fy</I>,<I>fz</I> = components of pairwise force
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<I>pN</I> = pair style specific quantities for allowed N values
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2009-12-22 07:01:58 +08:00
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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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2009-12-22 08:22:16 +08:00
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<PRE>compute 1 all pair/local eng
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2011-12-01 08:19:11 +08:00
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compute 1 all pair/local dist eng force
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compute 1 all pair/local dist eng fx fy fz
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compute 1 all pair/local dist fx fy fz p1 p2 p3
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2009-12-22 07:01:58 +08:00
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</PRE>
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<P><B>Description:</B>
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</P>
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<P>Define a computation that calculates properties of individual pairwise
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interactions. The number of datums generated, aggregated across all
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processors, equals the number of pairwise interactions in the system.
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2009-12-22 07:01:58 +08:00
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</P>
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2009-12-22 08:22:16 +08:00
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<P>The local data stored by this command is generated by looping over the
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pairwise neighbor list. Info about an individual pairwise interaction
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will only be included if both atoms in the pair are in the specified
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compute group, and if the current pairwise distance is less than the
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force cutoff distance for that interaction, as defined by the
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<A HREF = "pair_style.html">pair_style</A> and <A HREF = "pair_coeff.html">pair_coeff</A>
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commands.
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2009-12-22 07:01:58 +08:00
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</P>
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2011-12-01 08:19:11 +08:00
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<P>The output <I>dist</I> is the distance bewteen the pair of atoms.
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</P>
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<P>The output <I>eng</I> is the interaction energy for the pair of atoms.
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</P>
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<P>The output <I>force</I> is the force acting between the pair of atoms,
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which is positive for a repulsive force and negative for an attractive
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force. The outputs <I>fx</I>, <I>fy</I>, and <I>fz</I> are the xyz components of
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<I>force</I> on atom I.
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</P>
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<P>A pair style may define additional pairwise quantities which can be
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accessed as <I>p1</I> to <I>pN</I>, where N is defined by the pair style. Most
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pair styles do not define any additional quantities, so N = 0. An
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example of ones that do are the <A HREF = "pair_gran.html">granular pair styles</A>
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which calculate the tangential force between two particles and return
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its components and magnitude acting on atom I for N = 1,2,3,4. See
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individual pair styles for detils.
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</P>
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2009-12-22 07:01:58 +08:00
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<P>The output <I>dist</I> will be in distance <A HREF = "units.html">units</A>. The output
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<I>eng</I> will be in energy <A HREF = "units.html">units</A>. The outputs <I>force</I>,
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<I>fx</I>, <I>fy</I>, and <I>fz</I> will be in force <A HREF = "units.html">units</A>. The output
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<I>pN</I> will be in whatever units the pair style defines.
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2009-12-22 07:01:58 +08:00
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</P>
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<P>Note that as atoms migrate from processor to processor, there will be
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no consistent ordering of the entries within the local vector or array
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from one timestep to the next. The only consistency that is
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guaranteed is that the ordering on a particular timestep will be the
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same for local vectors or arrays generated by other compute commands.
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2009-12-22 08:22:16 +08:00
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For example, pair output from the <A HREF = "compute_property_local.html">compute
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property/local</A> command can be combined
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with data from this command and output by the <A HREF = "dump.html">dump local</A>
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command in a consistent way.
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</P>
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2011-04-12 03:49:41 +08:00
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<P>IMPORTANT NOTE: For pairs, if two atoms I,J are involved in 1-2, 1-3,
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1-4 interactions within the molecular topology, their pairwise
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interaction may be turned off, and thus they may not appear in the
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neighbor list, and will not be part of the local data created by this
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command. More specifically, this may be true of I,J pairs with a
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weighting factor of 0.0; pairs with a non-zero weighting factor are
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included. The weighting factors for 1-2, 1-3, and 1-4 pairwise
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interactions are set by the <A HREF = "special_bonds.html">special_bonds</A>
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command.
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</P>
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2009-12-22 07:01:58 +08:00
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<P><B>Output info:</B>
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</P>
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<P>This compute calculates a local vector or local array depending on the
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number of keywords. The length of the vector or number of rows in the
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array is the number of pairs. If a single keyword is specified, a
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local vector is produced. If two or more keywords are specified, a
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local array is produced where the number of columns = the number of
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keywords. The vector or array can be accessed by any command that
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2011-08-26 01:01:01 +08:00
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uses local values from a compute as input. See <A HREF = "Section_howto.html#howto_15">this
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section</A> for an overview of LAMMPS output
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options.
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</P>
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2010-01-24 07:20:05 +08:00
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<P>The output for <I>dist</I> will be in distance <A HREF = "units.html">units</A>. The
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output for <I>eng</I> will be in energy <A HREF = "units.html">units</A>. The output for
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<I>force</I> will be in force <A HREF = "units.html">units</A>.
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</P>
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2009-12-22 07:01:58 +08:00
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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 = "dump.html">dump local</A>, <A HREF = "compute_property_local.html">compute
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property/local</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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