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rLAMMPS lammps
compute_com.html
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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#4_15"
>
this section
</A>
for an overview of LAMMPS output options.
</P>
<P>
The vector values are "intensive", meaning they are independent of the
number of atoms in the simulation.
</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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