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<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 com command
</H3>
<P><B>Syntax:</B>
</P>
<PRE>compute ID group-ID com
</PRE>
<UL><LI>ID, group-ID are documented in <A HREF = "compute.html">compute</A> command
<LI>com = style name of this compute command
</UL>
<P><B>Examples:</B>
</P>
<PRE>compute 1 all com
</PRE>
<P><B>Description:</B>
</P>
<P>Define a computation that calculates the center-of-mass of the group
of atoms, including all effects due to atoms passing thru periodic
boundaries.
</P>
<P>A vector of three quantites is calculated by this compute, which
are the x,y,z coordinates of the center of mass.
</P>
<P>IMPORTANT NOTE: The coordinates of an atom contribute to the
center-of-mass 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>IMPORTANT NOTE: If an atom is part of a rigid body (see the <A HREF = "fix_rigid.html">fix
rigid</A> command), it's periodic image flags are altered,
and its contribution to the center-of-mass may not reflect its true
contribution. See the <A HREF = "fix_rigid.html">fix rigid</A> command for details.
Thus, to compute the center-of-mass of rigid bodies as they cross
periodic boundaries, you will need to post-process a <A HREF = "dump.html">dump
file</A> containing coordinates of the atoms in the bodies.
</P>
<P><B>Output info:</B>
</P>
<P>This compute calculates a global vector of length 3, which can be
accessed by indices 1-3 by any command that uses global vector values
from a compute as input. See <A HREF = "Section_howto.html#howto_15">this
section</A> for an overview of LAMMPS output
options.
</P>
<P>The vector values are "intensive". The vector values will be in
distance <A HREF = "units.html">units</A>.
</P>
<P><B>Restrictions:</B> none
</P>
<P><B>Related commands:</B>
</P>
<P><A HREF = "compute_com_molecule.html">compute com/molecule</A>
</P>
<P><B>Default:</B> none
</P>
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