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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>
</CENTER>
<HR>
<H3>compute omega/chunk command
</H3>
<P><B>Syntax:</B>
</P>
<PRE>compute ID group-ID omega/chunk chunkID
</PRE>
<UL><LI>ID, group-ID are documented in <A HREF = "compute.html">compute</A> command
<LI>omega/chunk = style name of this compute command
<LI>chunkID = ID of <A HREF = "compute_chunk_atom.html">compute chunk/atom</A> command
</UL>
<P><B>Examples:</B>
</P>
<PRE>compute 1 fluid omega/chunk molchunk
</PRE>
<P><B>Description:</B>
</P>
<P>Define a computation that calculates the angular velocity (omega) of
multiple chunks of atoms.
</P>
<P>In LAMMPS, chunks are collections of atoms defined by a <A HREF = "compute_chunk_atom.html">compute
chunk/atom</A> command, which assigns each atom
to a single chunk (or no chunk). The ID for this command is specified
as chunkID. For example, a single chunk could be the atoms in a
molecule or atoms in a spatial bin. See the <A HREF = "compute_chunk_atom.html">compute
chunk/atom</A> doc page and "<A HREF = "Section_howto.html#howto_23">Section_howto
23</A> for details of how chunks can be
defined and examples of how they can be used to measure properties of
a system.
</P>
<P>This compute calculates the 3 components of the angular velocity
vector for each chunk, via the formula L = Iw where L is the angular
momentum vector of the chunk, I is its moment of inertia tensor, and w
is omega = angular velocity of the chunk. The calculation includes
all effects due to atoms passing thru periodic boundaries.
</P>
<P>Note that only atoms in the specified group contribute to the
calculation. The <A HREF = "compute_chunk_atom.html">compute chunk/atom</A> command
defines its own group; atoms will have a chunk ID = 0 if they are not
in that group, signifying they are not assigned to a chunk, and will
thus also not contribute to this calculation. You can specify the
"all" group for this command if you simply want to include atoms with
non-zero chunk IDs.
</P>
<P>IMPORTANT NOTE: The coordinates of an atom contribute to the chunk's
angular velocity in "unwrapped" form, by using the image flags
associated with each atom. See the <A HREF = "dump.html">dump custom</A> command
for a discussion of "unwrapped" coordinates. See the Atoms section of
the <A HREF = "read_data.html">read_data</A> command for a discussion of image flags
and how they are set for each atom. You can reset the image flags
(e.g. to 0) before invoking this compute by using the <A HREF = "set.html">set
image</A> command.
</P>
<P>The simplest way to output the results of the compute omega/chunk
calculation to a file is to use the <A HREF = "fix_ave_time.html">fix ave/time</A>
command, for example:
</P>
<PRE>compute cc1 all chunk/atom molecule
compute myChunk all omega/chunk cc1
fix 1 all ave/time 100 1 100 c_myChunk file tmp.out mode vector
</PRE>
<P><B>Output info:</B>
</P>
<P>This compute calculates a global array where the number of rows = the
number of chunks <I>Nchunk</I> as calculated by the specified <A HREF = "compute_chunk_atom.html">compute
chunk/atom</A> command. The number of columns =
3 for the 3 xyz components of the angular velocity for each chunk.
These values can be accessed by any command that uses global array
values from a compute as input. See <A HREF = "Section_howto.html#howto_15">Section_howto
15</A> for an overview of LAMMPS output
options.
</P>
<P>The array values are "intensive". The array values will be in
velocity/distance <A HREF = "units.html">units</A>.
</P>
<P><B>Restrictions:</B> none
</P>
<P><B>Related commands:</B>
</P>
<P><A HREF = "variable.html">variable omega() function</A>
</P>
<P><B>Default:</B> none
</P>
</HTML>
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