forked from lijiext/lammps
91 lines
3.2 KiB
Plaintext
91 lines
3.2 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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fix setforce 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, assuming their initial velocity 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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[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. None of the "fix_modify"_fix_modify.html options
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are relevant to this fix.
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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"_Section_howto.html#4_15. This is the
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total force on the group of atoms before the forces on individual
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atoms are changed by the fix. The vector values calculated by this
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fix are "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. You should not
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specify force components with a variable that has time-dependence for
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use with a minimizer, since the minimizer increments the timestep as
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the iteration count during the 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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