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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>group command
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</H3>
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<P><B>Syntax:</B>
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</P>
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<PRE>group ID style args
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</PRE>
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<UL><LI>ID = user-defined name of the group
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<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>
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<PRE> <I>region</I> args = region-ID
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<I>type</I> or <I>id</I> or <I>molecule</I>
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args = one or more atom types, atom IDs, or molecule IDs
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args = logical value
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2007-06-20 20:56:17 +08:00
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logical = "<" or "<=" or ">" or ">=" or "==" or "!="
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value = an atom type or atom ID or molecule ID (depending on <I>style</I>)
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args = logical value1 value2
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logical = "<>"
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value1,value2 = atom types or atom IDs or molecule IDs
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(depending on <I>style</I>)
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<I>subtract</I> args = two or more group IDs
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<I>union</I> args = one or more group IDs
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<I>intersect</I> args = two or more group IDs
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</PRE>
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</UL>
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<P><B>Examples:</B>
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</P>
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<PRE>group edge region regstrip
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group water type 3 4
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group sub id <= 150
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group polyA molecule <> 50 250
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group boundary subtract all a2 a3
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group boundary union lower upper
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group boundary intersect upper flow
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</PRE>
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<P><B>Description:</B>
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</P>
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<P>Identify a collection of atoms as belonging to a group. The group ID
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can then be used in other commands such as fix, velocity, dump, or
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temperature to act on the atoms together.
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</P>
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<P>If the group ID already exists, the group command adds the specified
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atoms to the group.
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</P>
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<P>The <I>region</I> style puts all atoms in the region volume into the group.
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Note that this is a static one-time assignment. The atoms remain
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assigned (or not assigned) to the group even in they later move out of
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the region volume.
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</P>
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<P>The <I>type</I>, <I>id</I>, and <I>molecule</I> styles put all atoms with the
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specified atom types, atom IDs, or molecule IDs into the group. These
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3 styles can have their arguments specified in one of two formats.
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The 1st format is a list of values (types or IDs). For example, the
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2006-10-03 08:07:57 +08:00
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2nd command in the examples above puts all atoms of type 3 or 4 into
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the group named <I>water</I>. The 2nd format is a <I>logical</I> followed by
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one or two values (type or ID). The 7 valid logicals are listed
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above. All the logicals except <> take a single argument. The 3rd
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example above adds all atoms with IDs from 1 to 150 to the group named
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<I>sub</I>. The logical <> means "between" and takes 2 arguments. The 4th
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example above adds all atoms belonging to molecules with IDs from 50
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to 250 (inclusive) to the group named polyA.
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</P>
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<P>The <I>subtract</I> style takes a list of two or more existing group names
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as arguments. All atoms that belong to the 1st group, but not to any
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of the other groups are added to the specified group.
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</P>
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<P>The <I>union</I> style takes a list of one or more existing group names as
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arguments. All atoms that belong to any of the listed groups are
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added to the specified group.
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</P>
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<P>The <I>intersect</I> style takes a list of two or more existing group names
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as arguments. Atoms that belong to every one of the listed groups are
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added to the specified group.
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</P>
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<P>A group with the ID <I>all</I> is predefined. All atoms belong to this
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group.
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</P>
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<P><B>Restrictions:</B>
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</P>
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<P>There can be no more than 32 defined groups, including "all".
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</P>
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<P><B>Related commands:</B>
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</P>
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<P>region, fix, velocity, dump, temperature
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</P>
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<P><B>Default:</B>
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</P>
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<P>All atoms belong to the "all" group.
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</P>
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</HTML>
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