145 lines
4.8 KiB
ReStructuredText
145 lines
4.8 KiB
ReStructuredText
.. index:: fix setforce
|
|
.. index:: fix setforce/kk
|
|
.. index:: fix setforce/spin
|
|
|
|
fix setforce command
|
|
====================
|
|
|
|
Accelerator Variants: *setforce/kk*
|
|
|
|
fix setforce/spin command
|
|
=========================
|
|
|
|
Syntax
|
|
""""""
|
|
|
|
.. code-block:: LAMMPS
|
|
|
|
fix ID group-ID setforce fx fy fz keyword value ...
|
|
|
|
* ID, group-ID are documented in :doc:`fix <fix>` command
|
|
* setforce = style name of this fix command
|
|
* fx,fy,fz = force component values
|
|
* 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 freeze indenter setforce 0.0 0.0 0.0
|
|
fix 2 edge setforce NULL 0.0 0.0
|
|
fix 1 edge setforce/spin 0.0 0.0 0.0
|
|
fix 2 edge setforce NULL 0.0 v_oscillate
|
|
|
|
Description
|
|
"""""""""""
|
|
|
|
Set each component of force on each atom in the group to the specified
|
|
values fx,fy,fz. This erases all previously computed forces on the
|
|
atom, though additional fixes could add new forces. This command can
|
|
be used to freeze certain atoms in the simulation by zeroing their
|
|
force, either for running dynamics or performing an energy
|
|
minimization. For dynamics, this assumes their initial velocity is
|
|
also zero.
|
|
|
|
Any of the fx,fy,fz values can be specified as NULL which means do not
|
|
alter the force component in that dimension.
|
|
|
|
Any of the 3 quantities defining the force components can be specified
|
|
as an equal-style or atom-style :doc:`variable <variable>`, namely *fx*,
|
|
*fy*, *fz*\ . 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
|
|
force component.
|
|
|
|
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 force field.
|
|
|
|
Atom-style variables can specify the same formulas as equal-style
|
|
variables but can also include per-atom values, such as atom
|
|
coordinates. Thus it is easy to specify a spatially-dependent force
|
|
field with optional time-dependence as well.
|
|
|
|
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.
|
|
|
|
----------
|
|
|
|
Style *spin* suffix sets the components of the magnetic precession
|
|
vectors instead of the mechanical forces. This also erases all
|
|
previously computed magnetic precession vectors on the atom, though
|
|
additional magnetic fixes could add new forces.
|
|
|
|
This command can be used to freeze the magnetic moment of certain
|
|
atoms in the simulation by zeroing their precession vector.
|
|
|
|
All options defined above remain valid, they just apply to the magnetic
|
|
precession vectors instead of the forces.
|
|
|
|
----------
|
|
|
|
.. include:: accel_styles.rst
|
|
|
|
.. note::
|
|
|
|
The region keyword is supported by Kokkos, but a Kokkos-enabled
|
|
region must be used. See the region :doc:`region <region>` command for
|
|
more information.
|
|
|
|
----------
|
|
|
|
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 setting the forces to the desired values; on all
|
|
other levels, the force is set to 0.0 for the atoms in the fix group,
|
|
so that setforce values are not counted multiple times. Default is to
|
|
to override forces at the outermost level.
|
|
|
|
This fix computes a global 3-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, but you cannot set
|
|
forces to any value besides zero when performing a minimization. Use
|
|
the :doc:`fix addforce <fix_addforce>` command if you want to apply a
|
|
non-zero force to atoms during a minimization.
|
|
|
|
Restrictions
|
|
""""""""""""
|
|
|
|
Fix *setforce/spin* is part of the SPIN package. It is only
|
|
enabled if LAMMPS was built with that package. See the :doc:`Build
|
|
package <Build_package>` page for more info.
|
|
|
|
Related commands
|
|
""""""""""""""""
|
|
|
|
:doc:`fix addforce <fix_addforce>`, :doc:`fix aveforce <fix_aveforce>`
|
|
|
|
Default
|
|
"""""""
|
|
|
|
none
|