forked from lijiext/lammps
93 lines
3.5 KiB
HTML
93 lines
3.5 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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<HR>
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<H3>compute com/chunk 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 com/chunk chunkID
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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>com/chunk = style name of this compute command
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<LI>chunkID = ID of <A HREF = "compute_chunk_atom.html">compute chunk/atom</A> command
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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 fluid com/chunk molchunk
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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 center-of-mass for multiple
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chunks of atoms.
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</P>
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<P>In LAMMPS, chunks are collections of atoms defined by a <A HREF = "compute_chunk_atom.html">compute
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chunk/atom</A> command, which assigns each atom
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to a single chunk (or no chunk). The ID for this command is specified
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as chunkID. For example, a single chunk could be the atoms in a
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molecule or atoms in a spatial bin. See the <A HREF = "compute_chunk_atom.html">compute
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chunk/atom</A> doc page and "<A HREF = "Section_howto.html#howto_23">Section_howto
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23</A> for details of how chunks can be
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defined and examples of how they can be used to measure properties of
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a system.
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</P>
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<P>This compute calculates the x,y,z coordinates of the center-of-mass
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for each chunk, which includes all effects due to atoms passing thru
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periodic boundaries.
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</P>
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<P>Note that only atoms in the specified group contribute to the
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calculation. The <A HREF = "compute_chunk_atom.html">compute chunk/atom</A> command
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defines its own group; atoms will have a chunk ID = 0 if they are not
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in that group, signifying they are not assigned to a chunk, and will
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thus also not contribute to this calculation. You can specify the
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"all" group for this command if you simply want to include atoms with
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non-zero chunk IDs.
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</P>
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<P>IMPORTANT NOTE: The coordinates of an atom contribute to the chunk's
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center-of-mass in "unwrapped" form, by using the image flags
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associated with each atom. See the <A HREF = "dump.html">dump custom</A> command
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for a discussion of "unwrapped" coordinates. See the Atoms section of
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the <A HREF = "read_data.html">read_data</A> command for a discussion of image flags
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and how they are set for each atom. You can reset the image flags
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(e.g. to 0) before invoking this compute by using the <A HREF = "set.html">set
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image</A> command.
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</P>
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<P>The simplest way to output the results of the compute com/chunk
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calculation to a file is to use the <A HREF = "fix_ave_time.html">fix ave/time</A>
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command, for example:
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</P>
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<PRE>compute cc1 chunk/atom molecule
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compute myChunk all com/chunk cc1
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fix 1 all ave/time 100 1 100 c_myChunk file tmp.out mode vector
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</PRE>
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<P><B>Output info:</B>
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</P>
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<P>This compute calculates a global array where the number of rows = the
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number of chunks <I>Nchunk</I> as calculated by the specified <A HREF = "compute_chunk_atom.html">compute
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chunk/atom</A> command. The number of columns =
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3 for the x,y,z center-of-mass coordinates of each chunk. These
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values can be accessed by any command that uses global array values
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from a compute as input. See <A HREF = "Section_howto.html#howto_15">Section_howto
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15</A> for an overview of LAMMPS output
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options.
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</P>
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<P>The array values are "intensive". The array values will be in
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distance <A HREF = "units.html">units</A>.
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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 = "compute_com.html">compute com</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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