forked from lijiext/lammps
172 lines
7.0 KiB
Plaintext
172 lines
7.0 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 addforce command :h3
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fix addforce/cuda command :h3
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[Syntax:]
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fix ID group-ID addforce 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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addforce = 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) :pre
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zero or more keyword/value pairs may be appended to args :l
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keyword = {every} or {region} or {energy} :l
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{every} value = Nevery
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Nevery = add force every this many timesteps
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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
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{energy} value = v_name
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v_name = variable with name that calculates the potential energy of each atom in the added force field :pre
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:ule
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[Examples:]
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fix kick flow addforce 1.0 0.0 0.0
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fix kick flow addforce 1.0 0.0 v_oscillate
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fix ff boundary addforce 0.0 0.0 v_push energy v_espace :pre
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[Description:]
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Add fx,fy,fz to the corresponding component of force for each atom in
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the group. This command can be used to give an additional push to
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atoms in a simulation, such as for a simulation of Poiseuille flow in
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a channel.
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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(s) used to determine
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the 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 {every} keyword is used, the {Nevery} setting determines how
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often the forces are applied. The default value is 1, for every
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timestep.
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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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Adding a force to atoms implies a change in their potential energy as
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they move due to the applied force field. For dynamics via the "run"
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command, this energy can be optionally added to the system's potential
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energy for thermodynamic output (see below). For energy minimization
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via the "minimize" command, this energy must be added to the system's
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potential energy to formulate a self-consistent minimization problem
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(see below).
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The {energy} keyword is not allowed if the added force is a constant
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vector F = (fx,fy,fz), with all components defined as numeric
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constants and not as variables. This is because LAMMPS can compute
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the energy for each atom directly as E = -x dot F = -(x*fx + y*fy +
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z*fz), so that -Grad(E) = F.
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The {energy} keyword is optional if the added force is defined with
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one or more variables, and if you are performing dynamics via the
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"run"_run.html command. If the keyword is not used, LAMMPS will set
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the energy to 0.0, which is typically fine for dynamics.
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The {energy} keyword is required if the added force is defined with
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one or more variables, and you are performing energy minimization via
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the "minimize" command. The keyword specifies the name of an
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atom-style "variable"_variable.html which is used to compute the
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energy of each atom as function of its position. Like variables used
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for {fx}, {fy}, {fz}, the energy variable is specified as v_name,
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where name is the variable name.
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Note that when the {energy} keyword is used during an energy
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minimization, you must insure that the formula defined for the
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atom-style "variable"_variable.html is consistent with the force
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variable formulas, i.e. that -Grad(E) = F. For example, if the force
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were a spring-like F = kx, then the energy formula should be E =
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-0.5kx^2. If you don't do this correctly, the minimization will not
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converge properly.
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:line
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Styles with a {cuda} suffix are functionally the same as the
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corresponding style without the suffix. They have been optimized to
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run faster, depending on your available hardware, as discussed in
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"Section_accelerate"_Section_accelerate.html of the manual. The
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accelerated styles take the same arguments and should produce the same
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results, except for round-off and precision issues.
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These accelerated styles are part of the USER-CUDA package. They are
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only enabled if LAMMPS was built with that package. See the "Making
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LAMMPS"_Section_start.html#start_3 section 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"_Section_start.html#start_7 when you invoke LAMMPS, or you can
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use the "suffix"_suffix.html command in your input script.
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See "Section_accelerate"_Section_accelerate.html of the manual for
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more 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 {energy} option is supported by this
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fix to add the potential "energy" inferred by the added force to the
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system's potential energy as part of "thermodynamic
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output"_thermo_style.html. This is a fictitious quantity but is
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needed so that the "minimize"_minimize.html command can include the
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forces added by this fix in a consistent manner. I.e. there is a
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decrease in potential energy when atoms move in the direction of the
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added force.
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This fix computes a global scalar and a global 3-vector of forces,
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which can be accessed by various "output
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commands"_Section_howto.html#howto_15. The scalar is the potential
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energy discussed above. The vector is the total force on the group of
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atoms before the forces on individual atoms are changed by the fix.
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The scalar and vector values calculated by this 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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IMPORTANT NOTE: If you want the fictitious potential energy associated
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with the added forces to be included in the total potential energy of
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the system (the quantity being minimized), you MUST enable the
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"fix_modify"_fix_modify.html {energy} option for this fix.
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[Restrictions:] none
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[Related commands:]
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"fix setforce"_fix_setforce.html, "fix aveforce"_fix_aveforce.html
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[Default:]
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The option default for the every keyword is every = 1.
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