forked from lijiext/lammps
136 lines
4.5 KiB
Plaintext
136 lines
4.5 KiB
Plaintext
.. index:: fix aveforce
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fix aveforce command
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====================
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fix aveforce/cuda 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 aveforce fx fy fz keyword value ...
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* ID, group-ID are documented in :doc:`fix <fix>` command
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* aveforce = 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 = *region*
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.. parsed-literal::
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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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Examples
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""""""""
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.. parsed-literal::
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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
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Description
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"""""""""""
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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 :doc:`variable <variable>`, namely *fx*, *fy*, *fz*.
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If the value is a variable, it should be specified as v_name, where
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name is the variable name. In this case, the variable will be
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evaluated each timestep, and its value used to determine the average
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force.
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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 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 :doc:`region <region>` in order to have force added
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to it.
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----------
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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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: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 USER-CUDA package. They are
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only 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>`. None of the :doc:`fix_modify <fix_modify>` 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 :ref:`output commands <howto_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 :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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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 addforce <fix_addforce>`
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**Default:** none
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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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