forked from lijiext/lammps
124 lines
4.7 KiB
Plaintext
124 lines
4.7 KiB
Plaintext
"LAMMPS WWW Site"_lws - "LAMMPS Documentation"_ld - "LAMMPS Commands"_lc :c
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:link(lws,http://lammps.sandia.gov)
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:link(ld,Manual.html)
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:link(lc,Section_commands.html#comm)
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:line
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compute group/group command :h3
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[Syntax:]
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compute ID group-ID group/group group2-ID keyword value ... :pre
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ID, group-ID are documented in "compute"_compute.html command :ulb,l
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group/group = style name of this compute command :l
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group2-ID = group ID of second (or same) group :l
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zero or more keyword/value pairs may be appended :l
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keyword = {pair} or {kspace} or {boundary} :l
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{pair} value = {yes} or {no}
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{kspace} value = {yes} or {no}
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{boundary} value = {yes} or {no} :pre
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:ule
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[Examples:]
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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
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[Description:]
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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.
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If the {pair} keyword is set to {yes}, 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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interactions with atoms in the specified group2.
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If the {kspace} keyword is set to {yes}, which is not the default, and
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if a "kspace_style"_kspace_style.html 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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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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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 "(Bogusz)"_#Bogusz. If the
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{boundary} keyword is set to {yes}, which is the default, and {kspace}
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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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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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:line
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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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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 {pair} and {kspace} keywords. This is no more
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costly than using a single compute with both keywords set to {yes}.
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The individual contributions can be summed in a
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"variable"_variable.html if desired.
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This "document"_PDF/kspace.pdf describes how the long-range
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group-group calculations are performed.
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:line
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[Output info:]
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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 "this
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section"_Section_howto.html#howto_15 for an overview of LAMMPS output
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options.
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Both the scalar and vector values calculated by this compute are
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"extensive". The scalar value will be in energy "units"_units.html.
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The vector values will be in force "units"_units.html.
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[Restrictions:]
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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 "Tersoff"_pair_tersoff.html and
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"Stillinger-Weber"_pair_sw.html cannot be used. "EAM"_pair_eam.html
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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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Not all Kspace styles support calculation of group/group interactions.
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The {ewald} and {pppm} styles do.
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[Related commands:] none
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[Default:]
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The option defaults are pair = yes, kspace = no, and boundary = yes.
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:line
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:link(Bogusz)
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Bogusz et al, J Chem Phys, 108, 7070 (1998)
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