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rLAMMPS lammps
compute_gyration_molecule.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 gyration/molecule command
</H3>
<P><B>
Syntax:
</B>
</P>
<PRE>
compute ID group-ID gyration/molecule
</PRE>
<UL><LI>
ID, group-ID are documented in
<A
HREF =
"compute.html"
>
compute
</A>
command
<LI>
gyration/molecule = style name of this compute command
</UL>
<P><B>
Examples:
</B>
</P>
<PRE>
compute 1 molecule gyration/molecule
</PRE>
<P><B>
Description:
</B>
</P>
<P>
Define a computation that calculates the radius of gyration Rg of
individual molecules. The calculation includes all effects due to
atoms passing thru periodic boundaries.
</P>
<P>
Rg is a measure of the size of a molecule, and is computed by this
formula
</P>
<CENTER><IMG
SRC =
"Eqs/compute_gyration.jpg"
>
</CENTER>
<P>
where M is the total mass of the molecule, Rcm is the center-of-mass
position of the molecule, and the sum is over all atoms in the
molecule and in the group.
</P>
<P>
Rg for 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 Rg 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, them molecule IDs will be
in ascending order for any molecule with one or more of its atoms in
the specified group.
</P>
<P>
IMPORTANT NOTE: The coordinates of an atom contribute to Rg 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><B>
Output info:
</B>
</P>
<P>
This compute calculates a global vector of Rg values where the length
of the vector = Nmolecules. These values can be used 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 calculated by this compute 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>
none
</P>
<P><A
HREF =
"compute_gyration.html"
>
compute gyration
</A>
</P>
<P><B>
Default:
</B>
none
</P>
</HTML>
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