.. index:: fix aveforce

fix aveforce command
====================

Syntax
""""""

.. code-block:: LAMMPS

   fix ID group-ID aveforce fx fy fz keyword value ...

* ID, group-ID are documented in :doc:`fix <fix>` command
* aveforce = style name of this fix command
* fx,fy,fz = force component values (force units)

  .. parsed-literal::

       any of fx,fy,fz can be a variable (see below)

* zero or more keyword/value pairs may be appended to args
* keyword = *region*

  .. parsed-literal::

       *region* value = region-ID
         region-ID = ID of region atoms must be in to have added force

Examples
""""""""

.. code-block:: LAMMPS

   fix pressdown topwall aveforce 0.0 -1.0 0.0
   fix 2 bottomwall aveforce NULL -1.0 0.0 region top
   fix 2 bottomwall aveforce NULL -1.0 v_oscillate region top

Description
"""""""""""

Apply an additional external force to a group of atoms in such a way
that every atom experiences the same force.  This is useful for
pushing on wall or boundary atoms so that the structure of the wall
does not change over time.

The existing force is averaged for the group of atoms, component by
component.  The actual force on each atom is then set to the average
value plus the component specified in this command.  This means each
atom in the group receives the same force.

Any of the *fx*, *fy*, or *fz* values can be specified as :code:`NULL`, which
means the force in that dimension is not changed.  Note that this is not the
same as specifying a 0.0 value, since that sets all forces to the same
average value without adding in any additional force.

Any of the three quantities defining the force components, namely *fx*, *fy*,
and *fz*, can be specified as an equal-style :doc:`variable <variable>`\ .
If the value is a variable, it should be specified as v_name, where
name is the variable name.  In this case, the variable will be
evaluated each timestep, and its value used to determine the average
force.

Equal-style variables can specify formulas with various mathematical
functions, and include :doc:`thermo_style <thermo_style>` command
keywords for the simulation box parameters and timestep and elapsed
time.  Thus it is easy to specify a time-dependent average force.

If the *region* keyword is used, the atom must also be in the
specified geometric :doc:`region <region>` in order to have force added
to it.

----------

Restart, fix_modify, output, run start/stop, minimize info
"""""""""""""""""""""""""""""""""""""""""""""""""""""""""""

No information about this fix is written to
:doc:`binary restart files <restart>`.

The :doc:`fix_modify <fix_modify>` *respa* option is supported by this
fix. This allows to set at which level of the :doc:`r-RESPA <run_style>`
integrator the fix is adding its forces. Default is the outermost level.

This fix computes a global three-vector of forces, which can be accessed
by various :doc:`output commands <Howto_output>`.  This is the total
force on the group of atoms before the forces on individual atoms are
changed by the fix.  The vector values calculated by this fix are "extensive".

No parameter of this fix can be used with the *start/stop* keywords of
the :doc:`run <run>` command.

The forces due to this fix are imposed during an energy minimization,
invoked by the :doc:`minimize <minimize>` command.  You should not
specify force components with a variable that has time-dependence for
use with a minimizer, since the minimizer increments the timestep as
the iteration count during the minimization.

Restrictions
""""""""""""
 none

Related commands
""""""""""""""""

:doc:`fix setforce <fix_setforce>`, :doc:`fix addforce <fix_addforce>`

Default
"""""""

none
