2008-02-15 07:24:11 +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 group/group command
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</H3>
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<P><B>Syntax:</B>
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</P>
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2012-05-19 05:07:14 +08:00
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<PRE>compute ID group-ID group/group group2-ID keyword value ...
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2008-02-15 07:24:11 +08:00
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</PRE>
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2012-05-19 05:07:14 +08:00
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<UL><LI>ID, group-ID are documented in <A HREF = "compute.html">compute</A> command
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<LI>group/group = style name of this compute command
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<LI>group2-ID = group ID of second (or same) group
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2012-05-19 05:07:14 +08:00
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<LI>zero or more keyword/value pairs may be appended
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2012-05-23 22:45:27 +08:00
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<LI>keyword = <I>pair</I> or <I>kspace</I> or <I>boundary</I>
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2012-05-19 05:07:14 +08:00
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<PRE> <I>pair</I> value = <I>yes</I> or <I>no</I>
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2012-05-23 22:45:27 +08:00
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<I>kspace</I> value = <I>yes</I> or <I>no</I>
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<I>boundary</I> value = <I>yes</I> or <I>no</I>
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2012-05-19 05:07:14 +08:00
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</PRE>
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2008-02-15 07:24:11 +08:00
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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 lower group/group upper
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2012-05-19 05:07:14 +08:00
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compute 1 lower group/group upper kspace yes
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2008-02-15 07:24:11 +08:00
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compute mine fluid group/group wall
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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 total energy and force
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interaction between two groups of atoms: the compute group and the
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2012-05-19 05:07:14 +08:00
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specified group2. The two groups can be the same.
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</P>
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<P>If the <I>pair</I> keyword is set to <I>yes</I>, which is the default, then the
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the interaction energy will include a pair component which is defined
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as the pairwise energy between all pairs of atoms where one atom in
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the pair is in the first group and the other is in the second group.
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Likewise, the interaction force calculated by this compute will
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include the force on the compute group atoms due to pairwise
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2008-02-15 07:24:11 +08:00
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interactions with atoms in the specified group2.
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</P>
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2012-05-19 05:07:14 +08:00
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<P>If the <I>kspace</I> keyword is set to <I>yes</I>, which is not the default, and
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2012-05-23 22:45:27 +08:00
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if a <A HREF = "kspace_style.html">kspace_style</A> is defined, then the interaction
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energy will include a Kspace component which is the long-range
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Coulombic energy between all the atoms in the first group and all the
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2012-05-24 21:14:18 +08:00
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atoms in the 2nd group. Likewise, the interaction force calculated by
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this compute will include the force on the compute group atoms due to
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long-range Coulombic interactions with atoms in the specified group2.
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</P>
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<P>Normally the long-range Coulombic energy converges only when the net
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charge of the unit cell is zero. However, one can assume the net
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charge of the system is neutralized by a uniform background plasma,
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and a correction to the system energy can be applied to reduce
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artifacts. For more information see <A HREF = "#Bogusz">(Bogusz)</A>. If the
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<I>boundary</I> keyword is set to <I>yes</I>, which is the default, and <I>kspace</I>
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contributions are included, then this energy correction term will be
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added to the total group-group energy. This correction term does not
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affect the force calculation and will be zero if one or both of the
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groups are charge neutral. This energy correction term is the same as
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that included in the regular Ewald and PPPM routines.
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2012-05-23 22:45:27 +08:00
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</P>
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2012-05-19 05:07:14 +08:00
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<P>This compute does not calculate any bond or angle or dihedral or
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improper interactions between atoms in the two groups.
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</P>
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<HR>
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<P>The pairwise contributions to the group-group interactions are
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calculated by looping over a neighbor list. The Kspace contribution
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to the group-group interactions require essentially the same amount of
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work (FFTs, Ewald summation) as computing long-range forces for the
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entire system. Thus it can be costly to invoke this compute too
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frequently.
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</P>
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<P>If you desire a breakdown of the interactions into a pairwise and
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Kspace component, simply invoke the compute twice with the appropriate
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yes/no settings for the <I>pair</I> and <I>kspace</I> keywords. This is no more
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costly than using a single compute with both keywords set to <I>yes</I>.
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The individual contributions can be summed in a
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<A HREF = "variable.html">variable</A> if desired.
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</P>
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2012-05-24 21:14:18 +08:00
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<P>This <A HREF = "PDF/kspace.pdf">document</A> describes how the long-range
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group-group calculations are performed.
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2008-02-15 07:24:11 +08:00
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</P>
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2012-05-19 06:16:27 +08:00
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<HR>
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2008-02-15 07:24:11 +08:00
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<P><B>Output info:</B>
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</P>
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2009-12-04 07:58:11 +08:00
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<P>This compute calculates a global scalar (the energy) and a global
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vector of length 3 (force), which can be accessed by indices 1-3.
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These values can be used by any command that uses global scalar or
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vector values from a compute as input. See <A HREF = "Section_howto.html#howto_15">this
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section</A> for an overview of LAMMPS output
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options.
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</P>
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<P>Both the scalar and vector values calculated by this compute are
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2011-05-23 22:35:19 +08:00
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"extensive". The scalar value will be in energy <A HREF = "units.html">units</A>.
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2010-03-03 05:51:16 +08:00
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The vector values will be in force <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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2008-02-15 07:24:11 +08:00
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<P><B>Restrictions:</B>
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</P>
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2012-05-19 05:07:14 +08:00
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<P>Not all pair styles can be evaluated in a pairwise mode as required by
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this compute. For example, 3-body and other many-body potentials,
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such as <A HREF = "pair_tersoff.html">Tersoff</A> and
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<A HREF = "pair_sw.html">Stillinger-Weber</A> cannot be used. <A HREF = "pair_eam.html">EAM</A>
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potentials only include the pair potential portion of the EAM
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interaction when used by this compute, not the embedding term.
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</P>
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<P>Not all Kspace styles support calculation of group/group interactions.
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The <I>ewald</I> and <I>pppm</I> styles do.
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2008-02-15 07:24:11 +08:00
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</P>
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<P><B>Related commands:</B> none
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</P>
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2012-05-19 05:07:14 +08:00
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<P><B>Default:</B>
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</P>
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2012-05-23 22:45:27 +08:00
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<P>The option defaults are pair = yes, kspace = no, and boundary = yes.
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</P>
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2012-05-24 21:14:18 +08:00
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<HR>
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<A NAME = "Bogusz"></A>
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<P>Bogusz et al, J Chem Phys, 108, 7070 (1998)
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</P>
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2008-02-15 07:24:11 +08:00
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</HTML>
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