2008-02-15 07:24:11 +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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<HR>
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<H3>compute group/group command
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</H3>
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<P><B>Syntax:</B>
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</P>
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<PRE>compute ID group-ID group/group group2-ID
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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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<LI>group/group = style name of this compute command
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<LI>group2-ID = group ID of second (or same) group
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</UL>
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<P><B>Examples:</B>
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</P>
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<PRE>compute 1 lower group/group upper
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compute mine fluid group/group wall
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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 the total energy and force
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interaction between two groups of atoms: the compute group and the
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specified group2. The two groups can be the same. The interaction
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energy is defined as the pairwise energy between all pairs of atoms
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where one atom in the pair is in the first group and the other is in
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the second group. Likewise, the interaction force calculated by this
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compute is the force on the compute group atoms due to pairwise
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interactions with atoms in the specified group2.
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</P>
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<P>The energy and force are calculated by looping over a neighbor list of
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pairwise interactions. Thus it can be inefficient to compute this
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quantity too frequently.
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</P>
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<P><B>Output info:</B>
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</P>
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2009-12-04 07:58:11 +08:00
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<P>This compute calculates a global scalar (the energy) and a global
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vector of length 3 (force), which can be accessed by indices 1-3.
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These values can be used by any command that uses global scalar or
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vector values from a compute as input. See <A HREF = "Section_howto.html#4_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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<P>Both the scalar and vector values calculated by this compute are
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"extensive", meaning they scale with the number of atoms in the
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simulation.
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2008-02-15 07:24:11 +08:00
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</P>
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<P><B>Restrictions:</B>
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</P>
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<P>Only pairwise interactions, as defined by the
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<A HREF = "pair_style.html">pair_style</A> command, are included in this
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calculation. Bond (angle, dihedral, etc) interactions between atoms
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in the two groups are not included. Long-range interactions due to a
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<A HREF = "kspace_style.html">kspace_style</A> command are also not included. Not
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all pair potentials can be evaluated in a pairwise mode as required by
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this compute. For example, 3-body potentials, such as
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<A HREF = "pair_tersoff.html">Tersoff</A> and <A HREF = "pair_sw.html">Stillinger-Weber</A> cannot
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be used. <A HREF = "pair_eam.html">EAM</A> potentials for metals only include the
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pair potential portion of the EAM interaction, not the embedding
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term.
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</P>
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<P><B>Related commands:</B> none
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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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