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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 vcm/molecule command
</H3>
<P><B>Syntax:</B>
</P>
<PRE>compute ID group-ID vcm/molecule
</PRE>
<UL><LI>ID, group-ID are documented in <A HREF = "compute.html">compute</A> command
<LI>vcm/molecule = style name of this compute command
</UL>
<P><B>Examples:</B>
</P>
<PRE>compute 1 fluid vcm/molecule
</PRE>
<P><B>Description:</B>
</P>
<P>Define a computation that calculates the center-of-mass velocity of
individual molecules. The x,y,z components of the velocity of each
molecule are computed. This is calcualted by summing mass*velocity
for each atom in the molecule and dividing the sum by the total mass
of the molecule.
</P>
<P>The velocity of a particular molecule is only computed if one or more
of its atoms are in the specified group. Normally all atoms in the
molecule should be in the group, however this is not required. LAMMPS
will warn you if this is not the case. Only atoms in the group
contribute to the velocity calculation for the molecule.
</P>
<P>The ordering of per-molecule quantities produced by this compute is
consistent with the ordering produced by other compute commands that
generate per-molecule datums. Conceptually, the molecule IDs will be
in ascending order for any molecule with one or more of its atoms in
the specified group.
</P>
<P><B>Output info:</B>
</P>
<P>This compute calculates a global array where the number of rows =
Nmolecules and the number of columns = 3 for the vx,vy,vz components
of the center-of-mass velocity of each molecule. 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
distance <A HREF = "units.html">units</A>.
</P>
<P><B>Restrictions:</B> none
</P>
<P><B>Related commands:</B>
</P>
<P><A HREF = "compute_atom_molecule.html">compute atom/molecule</A>
</P>
<P><B>Default:</B> none
</P>
</HTML>
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