127 lines
4.8 KiB
Plaintext
127 lines
4.8 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,Commands_all.html)
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:line
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fix setforce command :h3
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fix setforce/kk command :h3
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[Syntax:]
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fix ID group-ID setforce fx fy fz keyword value ... :pre
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ID, group-ID are documented in "fix"_fix.html command :ulb,l
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setforce = style name of this fix command :l
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fx,fy,fz = force component values :l
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any of fx,fy,fz can be a variable (see below) :l
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zero or more keyword/value pairs may be appended to args :l
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keyword = {region} :l
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{region} value = region-ID
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region-ID = ID of region atoms must be in to have added force :pre
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:ule
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[Examples:]
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fix freeze indenter setforce 0.0 0.0 0.0
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fix 2 edge setforce NULL 0.0 0.0
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fix 2 edge setforce NULL 0.0 v_oscillate :pre
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[Description:]
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Set each component of force on each atom in the group to the specified
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values fx,fy,fz. This erases all previously computed forces on the
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atom, though additional fixes could add new forces. This command can
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be used to freeze certain atoms in the simulation by zeroing their
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force, either for running dynamics or performing an energy
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minimization. For dynamics, this assumes their initial velocity is
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also zero.
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Any of the fx,fy,fz values can be specified as NULL which means do not
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alter the force component in that dimension.
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Any of the 3 quantities defining the force components can be specified
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as an equal-style or atom-style "variable"_variable.html, namely {fx},
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{fy}, {fz}. If the value is a variable, it should be specified as
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v_name, where name is the variable name. In this case, the variable
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will be evaluated each timestep, and its value used to determine the
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force component.
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Equal-style variables can specify formulas with various mathematical
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functions, and include "thermo_style"_thermo_style.html command
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keywords for the simulation box parameters and timestep and elapsed
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time. Thus it is easy to specify a time-dependent force field.
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Atom-style variables can specify the same formulas as equal-style
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variables but can also include per-atom values, such as atom
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coordinates. Thus it is easy to specify a spatially-dependent force
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field with optional time-dependence as well.
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If the {region} keyword is used, the atom must also be in the
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specified geometric "region"_region.html in order to have force added
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to it.
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:line
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Styles with a {gpu}, {intel}, {kk}, {omp}, or {opt} suffix are
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functionally the same as the corresponding style without the suffix.
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They have been optimized to run faster, depending on your available
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hardware, as discussed on the "Speed packages"_Speed_packages.html doc
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page. The accelerated styles take the same arguments and should
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produce the same results, except for round-off and precision issues.
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The region keyword is also supported by Kokkos, but a Kokkos-enabled
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region must be used. See the region "region"_region.html command for
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more information.
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These accelerated styles are part of the r Kokkos package. They are
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only enabled if LAMMPS was built with that package. See the "Build
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package"_Build_package.html doc page for more info.
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You can specify the accelerated styles explicitly in your input script
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by including their suffix, or you can use the "-suffix command-line
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switch"_Run_options.html when you invoke LAMMPS, or you can use the
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"suffix"_suffix.html command in your input script.
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See the "Speed packages"_Speed_packages.html doc page for more
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instructions on how to use the accelerated styles effectively.
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:line
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[Restart, fix_modify, output, run start/stop, minimize info:]
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No information about this fix is written to "binary restart
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files"_restart.html.
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The "fix_modify"_fix_modify.html {respa} option is supported by
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this fix. This allows to set at which level of the "r-RESPA"_run_style.html
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integrator the fix is setting the forces to the desired values; on all
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other levels, the force is set to 0.0 for the atoms in the fix group,
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so that setforce values are not counted multiple times. Default is to
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to override forces at the outermost level.
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This fix computes a global 3-vector of forces, which can be accessed
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by various "output commands"_Howto_output.html. This is the total
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force on the group of atoms before the forces on individual atoms are
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changed by the fix. The vector values calculated by this fix are
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"extensive".
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No parameter of this fix can be used with the {start/stop} keywords of
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the "run"_run.html command.
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The forces due to this fix are imposed during an energy minimization,
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invoked by the "minimize"_minimize.html command, but you cannot set
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forces to any value besides zero when performing a minimization. Use
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the "fix addforce"_fix_addforce.html command if you want to apply a
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non-zero force to atoms during a minimization.
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[Restrictions:] none
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[Related commands:]
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"fix addforce"_fix_addforce.html, "fix aveforce"_fix_aveforce.html
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[Default:] none
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