forked from lijiext/lammps
197 lines
7.4 KiB
Plaintext
197 lines
7.4 KiB
Plaintext
.. index:: fix addforce
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fix addforce command
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====================
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Syntax
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""""""
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.. parsed-literal::
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fix ID group-ID addforce fx fy fz keyword value ...
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* ID, group-ID are documented in :doc:`fix <fix>` command
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* addforce = style name of this fix command
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* fx,fy,fz = force component values (force units)
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.. parsed-literal::
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any of fx,fy,fz can be a variable (see below)
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* zero or more keyword/value pairs may be appended to args
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* keyword = *every* or *region* or *energy*
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.. parsed-literal::
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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
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Examples
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""""""""
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.. parsed-literal::
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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
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Description
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"""""""""""
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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 :doc:`variable <variable>`, 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 :doc:`thermo_style <thermo_style>` 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 :doc:`region <region>` in order to have force added
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to it.
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----------
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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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:doc:`run <run>` 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 :doc:`variable <variable>` 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 :doc:`variable <variable>` 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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----------
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Styles with a suffix are functionally the same as the corresponding
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style without the suffix. They have been optimized to run faster,
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depending on your available hardware, as discussed in
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:doc:`Section_accelerate <Section_accelerate>` 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 ackage. They are only
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enabled if LAMMPS was built with that package. See the :ref:`Making LAMMPS <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 :ref:`-suffix command-line switch <start_7>` when you invoke LAMMPS, or you can
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use the :doc:`suffix <suffix>` command in your input script.
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See :doc:`Section_accelerate <Section_accelerate>` of the manual for
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more instructions on how to use the accelerated styles effectively.
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----------
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Restart, fix_modify, output, run start/stop, minimize info
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""""""""""""""""""""""""""""""""""""""""""""""""""""""""""
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No information about this fix is written to :doc:`binary restart files <restart>`.
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The :doc:`fix_modify <fix_modify>` *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 :doc:`thermodynamic output <thermo_style>`. This is a fictitious quantity but is
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needed so that the :doc:`minimize <minimize>` 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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The :doc:`fix_modify <fix_modify>` *respa* option is supported by this
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fix. This allows to set at which level of the :doc:`r-RESPA <run_style>`
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integrator the fix is adding its forces. Default is the outermost
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level.
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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 :ref:`output commands <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 :doc:`run <run>` command.
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The forces due to this fix are imposed during an energy minimization,
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invoked by the :doc:`minimize <minimize>` 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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.. note::
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If you want the fictitious potential energy associated with the
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added forces to be included in the total potential energy of the
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system (the quantity being minimized), you MUST enable the
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:doc:`fix_modify <fix_modify>` *energy* option for this fix.
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Restrictions
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""""""""""""
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none
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Related commands
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""""""""""""""""
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:doc:`fix setforce <fix_setforce>`, :doc:`fix aveforce <fix_aveforce>`
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Default
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"""""""
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The option default for the every keyword is every = 1.
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.. _lws: http://lammps.sandia.gov
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.. _ld: Manual.html
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.. _lc: Section_commands.html#comm
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