115 lines
3.6 KiB
ReStructuredText
115 lines
3.6 KiB
ReStructuredText
.. 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.
|
|
|
|
----------
|
|
|
|
.. include:: accel_styles.rst
|
|
|
|
----------
|
|
|
|
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
|