2015-02-14 00:54:23 +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 property/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 property/chunk chunkID input1 input2 ...
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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>property/chunk = style name of this compute command
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<LI>input = one or more attributes
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<PRE> attributes = count, id, coord1, coord2, coord3
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count = # of atoms in chunk
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id = original chunk IDs before compression by <A HREF = "compute_chunk_atom.html">compute chunk/atom</A>
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coord123 = coordinates for spatial bins calculated by <A HREF = "compute_chunk_atom.html">compute chunk/atom</A>
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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>compute 1 all property/chunk count
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compute 1 all property/chunk ID coord1
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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 stores the specified attributes of chunks of
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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 and stores the specified attributes of chunks
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as global data so they can be accessed by other <A HREF = "Section_howto.html#howto_15">output
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commands</A> and used in conjunction with
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other commands that generate per-chunk data, such as <A HREF = "compute_com_chunk.html">compute
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com/chunk</A> or <A HREF = "compute_msd_chunk.html">compute
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msd/chunk</A>.
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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 of the <I>count</I> attribute. The <A HREF = "compute_chunk_atom.html">compute
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chunk/atom</A> command defines its own group;
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atoms will have a chunk ID = 0 if they are not in that group,
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signifying they are not assigned to a chunk, and will thus also not
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contribute to this calculation. You can specify the "all" group for
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this command if you simply want to include atoms with non-zero chunk
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IDs.
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</P>
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<P>The <I>count</I> attribute is the number of atoms in the chunk.
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</P>
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<P>The <I>id</I> attribute stores the original chunk ID for each chunk. It
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can only be used if the <I>compress</I> keyword was set to <I>yes</I> for the
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<A HREF = "compute_chunk_atom.html">compute chunk/atom</A> command referenced by
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chunkID. This means that the original chunk IDs (e.g. molecule IDs)
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will have been compressed to remove chunk IDs with no atoms assigned
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to them. Thus a compresed chunk ID of 3 may correspond to an original
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chunk ID (molecule ID in this case) of 415. The <I>id</I> attribute will
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then be 415 for the 3rd chunk.
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</P>
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<P>The <I>coordN</I> attributes can only be used if a <I>binning</I> style was used
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in the <A HREF = "compute_chunk_atom.html">compute chunk/atom</A> command referenced
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by chunkID. For <I>bin/1d</I>, <I>bin/2d</I>, and <I>bin/3d</I> styles the attribute
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is the center point of the bin in the corresponding dimension. Style
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<I>bin/1d</I> only defines a <I>coord1</I> attribute. Style <I>bin/2d</I> adds a
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<I>coord2</I> attribute. Style <I>bin/3d</I> adds a <I>coord3</I> attribute.
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</P>
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<P>Note that if the value of the <I>units</I> keyword used in the <A HREF = "compute_chunk_atom.html">compute
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chunk/atom command</A> is <I>box</I> or <I>lattice</I>, the
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<I>coordN</I> attributes will be in distance <A HREF = "units.html">units</A>. If the
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value of the <I>units</I> keyword is <I>reduced</I>, the <I>coordN</I> attributes
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will be in unitless reduced units (0-1).
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</P>
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<P>The simplest way to output the results of the compute property/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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2015-02-14 04:28:48 +08:00
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<PRE>compute cc1 all chunk/atom molecule
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2015-02-14 00:54:23 +08:00
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compute myChunk1 all property/chunk cc1
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compute myChunk2 all com/chunk cc1
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fix 1 all ave/time 100 1 100 c_myChunk1 c_myChunk2 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 vector or global array depending on
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the number of input values. The length of the vector or number of
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rows in the array is the number of chunks.
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</P>
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<P>This compute calculates a global vector or global array where the
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number of rows = the number of chunks <I>Nchunk</I> as calculated by the
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specified <A HREF = "compute_chunk_atom.html">compute chunk/atom</A> command. If a
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single input is specified, a global vector is produced. If two or
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more inputs are specified, a global array is produced where the number
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of columns = the number of inputs. The vector or array can be
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accessed by any command that uses global values from a compute as
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input. See <A HREF = "Section_howto.html#howto_15">this section</A> for an overview
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of LAMMPS output options.
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</P>
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<P>The vector or array values are "intensive". The values will be
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unitless or in the units discussed above.
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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 = "fix_ave_chunk.html">fix ave/chunk</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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