2009-12-04 08:42:15 +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 com 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 com
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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>com = style name of this compute command
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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 com
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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 calculates the center-of-mass of the group
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of atoms, including all effects due to atoms passing thru periodic
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boundaries.
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</P>
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<P>A vector of three quantites is calculated by this compute, which
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are the x,y,z coordinates of the center of mass.
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</P>
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<P>IMPORTANT NOTE: The coordinates of an atom contribute to the
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center-of-mass in "unwrapped" form, by using the image flags
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associated with each atom. See the <A HREF = "dump.html">dump custom</A> command
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for a discussion of "unwrapped" coordinates. See the Atoms section of
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the <A HREF = "read_data.html">read_data</A> command for a discussion of image flags
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and how they are set for each atom. You can reset the image flags
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(e.g. to 0) before invoking this compute by using the <A HREF = "set.html">set
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image</A> command.
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</P>
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<P>IMPORTANT NOTE: If an atom is part of a rigid body (see the <A HREF = "fix_rigid.html">fix
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rigid</A> command), it's periodic image flags are altered,
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and its contribution to the center-of-mass may not reflect its true
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contribution. See the <A HREF = "fix_rigid.html">fix rigid</A> command for details.
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Thus, to compute the center-of-mass of rigid bodies as they cross
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periodic boundaries, you will need to post-process a <A HREF = "dump.html">dump
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file</A> containing coordinates of the atoms in the bodies.
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</P>
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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 of length 3, which can be
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accessed by indices 1-3 by any command that uses global vector values
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from a compute as input. See <A HREF = "Section_howto.html#4_15">this section</A>
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for an overview of LAMMPS output options.
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</P>
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2010-03-03 05:51:16 +08:00
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<P>The vector values are "intensive". The vector values will be in
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distance <A HREF = "units.html">units</A>.
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2010-01-24 07:20:05 +08:00
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</P>
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2009-12-04 08:42:15 +08:00
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<P><B>Restrictions:</B> none
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</P>
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2009-12-22 01:34:52 +08:00
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<P><B>Related commands:</B>
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</P>
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<P><A HREF = "compute_com_molecule.html">compute com/molecule</A>
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2009-12-04 08:42:15 +08:00
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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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