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< title > compute group/group command — LAMMPS documentation< / title >
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< li class = "toctree-l1" > < a class = "reference internal" href = "Section_intro.html" > 1. Introduction< / a > < / li >
< li class = "toctree-l1" > < a class = "reference internal" href = "Section_start.html" > 2. Getting Started< / a > < / li >
< li class = "toctree-l1" > < a class = "reference internal" href = "Section_commands.html" > 3. Commands< / a > < / li >
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< li class = "toctree-l1" > < a class = "reference internal" href = "Section_accelerate.html" > 5. Accelerating LAMMPS performance< / a > < / li >
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< li class = "toctree-l1" > < a class = "reference internal" href = "Section_tools.html" > 9. Additional tools< / a > < / li >
< li class = "toctree-l1" > < a class = "reference internal" href = "Section_modify.html" > 10. Modifying & extending LAMMPS< / a > < / li >
< li class = "toctree-l1" > < a class = "reference internal" href = "Section_python.html" > 11. Python interface to LAMMPS< / a > < / li >
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< li > compute group/group command< / li >
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< a href = "http://lammps.sandia.gov" > Website< / a >
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< span id = "index-0" > < / span > < h1 > compute group/group command< a class = "headerlink" href = "#compute-group-group-command" title = "Permalink to this headline" > ¶< / a > < / h1 >
< div class = "section" id = "syntax" >
< h2 > Syntax< a class = "headerlink" href = "#syntax" title = "Permalink to this headline" > ¶< / a > < / h2 >
< div class = "highlight-python" > < div class = "highlight" > < pre > compute ID group-ID group/group group2-ID keyword value ...
< / pre > < / div >
< / div >
< ul class = "simple" >
< li > ID, group-ID are documented in < a class = "reference internal" href = "compute.html" > < em > compute< / em > < / a > command< / li >
< li > group/group = style name of this compute command< / li >
< li > group2-ID = group ID of second (or same) group< / li >
< li > zero or more keyword/value pairs may be appended< / li >
< li > keyword = < em > pair< / em > or < em > kspace< / em > or < em > boundary< / em > < / li >
< / ul >
< pre class = "literal-block" >
< em > pair< / em > value = < em > yes< / em > or < em > no< / em >
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< em > kspace< / em > value = < em > yes< / em > or < em > no< / em >
< em > boundary< / em > value = < em > yes< / em > or < em > no< / em >
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< / pre >
< / div >
< div class = "section" id = "examples" >
< h2 > Examples< a class = "headerlink" href = "#examples" title = "Permalink to this headline" > ¶< / a > < / h2 >
< div class = "highlight-python" > < div class = "highlight" > < pre > compute 1 lower group/group upper
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compute 1 lower group/group upper kspace yes
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compute mine fluid group/group wall
< / pre > < / div >
< / div >
< / div >
< div class = "section" id = "description" >
< h2 > Description< a class = "headerlink" href = "#description" title = "Permalink to this headline" > ¶< / a > < / h2 >
< 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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specified group2. The two groups can be the same.< / p >
< p > If the < em > pair< / em > keyword is set to < em > yes< / em > , which is the default, then the
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the interaction energy will include a pair component which is defined
as the pairwise energy between all pairs of atoms where one atom in
the pair is in the first group and the other is in the second group.
Likewise, the interaction force calculated by this compute will
include the force on the compute group atoms due to pairwise
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interactions with atoms in the specified group2.< / p >
< p > If the < em > kspace< / em > keyword is set to < em > yes< / em > , which is not the default, and
if a < a class = "reference internal" href = "kspace_style.html" > < em > kspace_style< / em > < / a > is defined, then the interaction
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energy will include a Kspace component which is the long-range
Coulombic energy between all the atoms in the first group and all the
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atoms in the 2nd group. Likewise, the interaction force calculated by
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.< / p >
< 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
charge of the system is neutralized by a uniform background plasma,
and a correction to the system energy can be applied to reduce
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artifacts. For more information see < a class = "reference internal" href = "#bogusz" > < span > (Bogusz)< / span > < / a > . If the
< em > boundary< / em > keyword is set to < em > yes< / em > , which is the default, and < em > kspace< / em >
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contributions are included, then this energy correction term will be
added to the total group-group energy. This correction term does not
affect the force calculation and will be zero if one or both of the
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.< / p >
< p > This compute does not calculate any bond or angle or dihedral or
improper interactions between atoms in the two groups.< / p >
< hr class = "docutils" / >
< p > The pairwise contributions to the group-group interactions are
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calculated by looping over a neighbor list. The Kspace contribution
to the group-group interactions require essentially the same amount of
work (FFTs, Ewald summation) as computing long-range forces for the
entire system. Thus it can be costly to invoke this compute too
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frequently.< / p >
< 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 < em > pair< / em > and < em > kspace< / em > keywords. This is no more
costly than using a single compute with both keywords set to < em > yes< / em > .
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The individual contributions can be summed in a
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< a class = "reference internal" href = "variable.html" > < em > variable< / em > < / a > if desired.< / p >
< p > This < a class = "reference external" href = "PDF/kspace.pdf" > document< / a > describes how the long-range
group-group calculations are performed.< / p >
< hr class = "docutils" / >
< p > < strong > Output info:< / strong > < / p >
< 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.
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 class = "reference internal" href = "Section_howto.html#howto-15" > < span > this section< / span > < / a > for an overview of LAMMPS output
options.< / p >
< p > Both the scalar and vector values calculated by this compute are
“ extensive” . The scalar value will be in energy < a class = "reference internal" href = "units.html" > < em > units< / em > < / a > .
The vector values will be in force < a class = "reference internal" href = "units.html" > < em > units< / em > < / a > .< / p >
< / div >
< div class = "section" id = "restrictions" >
< h2 > Restrictions< a class = "headerlink" href = "#restrictions" title = "Permalink to this headline" > ¶< / a > < / h2 >
< 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 class = "reference internal" href = "pair_tersoff.html" > < em > Tersoff< / em > < / a > and
< a class = "reference internal" href = "pair_sw.html" > < em > Stillinger-Weber< / em > < / a > cannot be used. < a class = "reference internal" href = "pair_eam.html" > < em > EAM< / em > < / 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.< / p >
< p > Not all Kspace styles support calculation of group/group interactions.
The < em > ewald< / em > and < em > pppm< / em > styles do.< / p >
< p > < strong > Related commands:< / strong > none< / p >
< / div >
< div class = "section" id = "default" >
< h2 > Default< a class = "headerlink" href = "#default" title = "Permalink to this headline" > ¶< / a > < / h2 >
< p > The option defaults are pair = yes, kspace = no, and boundary = yes.< / p >
< hr class = "docutils" / >
< p id = "bogusz" > Bogusz et al, J Chem Phys, 108, 7070 (1998)< / p >
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