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rLAMMPS lammps
compute_vcm_chunk.html
View Options
<HTML>
<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 vcm/chunk command
</H3>
<P><B>
Syntax:
</B>
</P>
<PRE>
compute ID group-ID vcm/chunk chunkID
</PRE>
<UL><LI>
ID, group-ID are documented in
<A
HREF =
"compute.html"
>
compute
</A>
command
<LI>
vcm/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 vcm/chunk molchunk
</PRE>
<P><B>
Description:
</B>
</P>
<P>
Define a computation that calculates the center-of-mass velocity for
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 x,y,z components of the center-of-mass
velocity for each chunk. This is done by summing mass*velocity for
each atom in the chunk and dividing the sum by the total mass of the
chunk.
</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>
The simplest way to output the results of the compute vcm/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 vcm/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 x,y,z center-of-mass velocity coordinates of 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
<A
HREF =
"units.html"
>
units
</A>
.
</P>
<P><B>
Restrictions:
</B>
none
</P>
<P><B>
Related commands:
</B>
none
</P>
<P><B>
Default:
</B>
none
</P>
</HTML>
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