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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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<H3>group command
</H3>
<P><B>Syntax:</B>
</P>
<PRE>group ID style args
</PRE>
<UL><LI>ID = user-defined name of the group
<LI>style = <I>region</I> or <I>type</I> or <I>id</I> or <I>molecule</I> or <I>subtract</I> or <I>union</I> or <I>intersect</I>
<PRE> <I>region</I> args = region-ID
<I>type</I> or <I>id</I> or <I>molecule</I>
args = one or more atom types, atom IDs, or molecule IDs
args = logical value
logical = "<" or "<=" or ">" or ">=" or "==" or "!="
value = an atom type or atom ID or molecule ID (depending on <I>style</I>)
args = logical value1 value2
logical = "<>"
value1,value2 = atom types or atom IDs or molecule IDs
(depending on <I>style</I>)
<I>subtract</I> args = two or more group IDs
<I>union</I> args = one or more group IDs
<I>intersect</I> args = two or more group IDs
</PRE>
</UL>
<P><B>Examples:</B>
</P>
<PRE>group edge region regstrip
group water type 3 4
group sub id <= 150
group polyA molecule <> 50 250
group boundary subtract all a2 a3
group boundary union lower upper
group boundary intersect upper flow
</PRE>
<P><B>Description:</B>
</P>
<P>Identify a collection of atoms as belonging to a group. The group ID
can then be used in other commands such as fix, velocity, dump, or
temperature to act on the atoms together.
</P>
<P>If the group ID already exists, the group command adds the specified
atoms to the group.
</P>
<P>The <I>region</I> style puts all atoms in the region volume into the group.
Note that this is a static one-time assignment. The atoms remain
assigned (or not assigned) to the group even in they later move out of
the region volume.
</P>
<P>The <I>type</I>, <I>id</I>, and <I>molecule</I> styles put all atoms with the
specified atom types, atom IDs, or molecule IDs into the group. These
3 styles can have their arguments specified in one of two formats.
The 1st format is a list of values (types or IDs). For example, the
2nd command in the examples above puts all atoms of type 3 or 4 into
the group named <I>water</I>. The 2nd format is a <I>logical</I> followed by
one or two values (type or ID). The 7 valid logicals are listed
above. All the logicals except <> take a single argument. The 3rd
example above adds all atoms with IDs from 1 to 150 to the group named
<I>sub</I>. The logical <> means "between" and takes 2 arguments. The 4th
example above adds all atoms belonging to molecules with IDs from 50
to 250 (inclusive) to the group named polyA.
</P>
<P>The <I>subtract</I> style takes a list of two or more existing group names
as arguments. All atoms that belong to the 1st group, but not to any
of the other groups are added to the specified group.
</P>
<P>The <I>union</I> style takes a list of one or more existing group names as
arguments. All atoms that belong to any of the listed groups are
added to the specified group.
</P>
<P>The <I>intersect</I> style takes a list of two or more existing group names
as arguments. Atoms that belong to every one of the listed groups are
added to the specified group.
</P>
<P>A group with the ID <I>all</I> is predefined. All atoms belong to this
group.
</P>
<P><B>Restrictions:</B>
</P>
<P>There can be no more than 32 defined groups, including "all".
</P>
<P><B>Related commands:</B>
</P>
<P>region, fix, velocity, dump, temperature
</P>
<P><B>Default:</B>
</P>
<P>All atoms belong to the "all" group.
</P>
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