forked from lijiext/lammps
91 lines
3.3 KiB
Plaintext
91 lines
3.3 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 aveforce command :h3
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[Syntax:]
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fix ID group-ID aveforce 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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aveforce = style name of this fix command :l
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fx,fy,fz = force component values (force units) :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 pressdown topwall aveforce 0.0 -1.0 0.0
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fix 2 bottomwall aveforce NULL -1.0 0.0 region top
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fix 2 bottomwall aveforce NULL -1.0 v_oscillate region top :pre
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[Description:]
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Apply an additional external force to a group of atoms in such a way
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that every atom experiences the same force. This is useful for
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pushing on wall or boundary atoms so that the structure of the wall
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does not change over time.
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The existing force is averaged for the group of atoms, component by
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component. The actual force on each atom is then set to the average
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value plus the component specified in this command. This means each
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atom in the group receives the same force.
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Any of the fx,fy,fz values can be specified as NULL which means the
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force in that dimension is not changed. Note that this is not the
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same as specifying a 0.0 value, since that sets all forces to the same
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average value without adding in any additional force.
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Any of the 3 quantities defining the force components can be specified
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as an equal-style "variable"_variable.html, namely {fx}, {fy}, {fz}.
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If the value is a variable, it should be specified as v_ID, where ID
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is the variable ID. In this case, the variable will be evaluated each
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timestep, and its value used to determine the average force.
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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 average force.
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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 setforce"_fix_setforce.html, "fix addforce"_fix_addforce.html
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[Default:] none
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