.. index:: fix addforce

fix addforce command
====================

Syntax
""""""

.. code-block:: LAMMPS

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

* ID, group-ID are documented in :doc:`fix <fix>` command
* addforce = 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 = *every* or *region* or *energy*

  .. parsed-literal::

       *every* value = Nevery
         Nevery = add force every this many time steps
       *region* value = region-ID
         region-ID = ID of region atoms must be in to have added force
       *energy* value = v_name
         v_name = variable with name that calculates the potential energy of each atom in the added force field

Examples
""""""""

.. code-block:: LAMMPS

   fix kick flow addforce 1.0 0.0 0.0
   fix kick flow addforce 1.0 0.0 v_oscillate
   fix ff boundary addforce 0.0 0.0 v_push energy v_espace

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

Add :math:`(f_x,f_y,f_z)` to the corresponding component of the force for each
atom in the group.  This command can be used to give an additional push to
atoms in a simulation, such as for a simulation of Poiseuille flow in
a channel.

Any of the three quantities defining the force components, namely :math:`f_x`,
:math:`f_y`, and :math:`f_z`, can be specified as an equal-style or atom-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 time step, and its value(s) will be used to determine
the force component(s).

Equal-style variables can specify formulas with various mathematical
functions and include :doc:`thermo_style <thermo_style>` command
keywords for the simulation box parameters, time step, 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 *every* keyword is used, the *Nevery* setting determines how
often the forces are applied.  The default value is 1, for every
time step.

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.

----------

Adding a force to atoms implies a change in their potential energy as
they move due to the applied force field.  For dynamics via the "run"
command, this energy can be optionally added to the system's potential
energy for thermodynamic output (see below).  For energy minimization
via the "minimize" command, this energy must be added to the system's
potential energy to formulate a self-consistent minimization problem
(see below).

The *energy* keyword is not allowed if the added force is a constant
vector :math:`\vec F = (f_x,f_y,f_z)`, with all components defined as numeric
constants and not as variables.  This is because LAMMPS can compute
the energy for each atom directly as

.. math::
   E = -\vec x \cdot \vec F = -(x f_x + y f_y + z f_z),

so that :math:`-\vec\nabla E = \vec F`.

The *energy* keyword is optional if the added force is defined with
one or more variables, and if you are performing dynamics via the
:doc:`run <run>` command.  If the keyword is not used, LAMMPS will set
the energy to 0.0, which is typically fine for dynamics.

The *energy* keyword is required if the added force is defined with
one or more variables, and you are performing energy minimization via
the "minimize" command.  The keyword specifies the name of an
atom-style :doc:`variable <variable>` which is used to compute the
energy of each atom as function of its position.  Like variables used
for :math:`f_x`, :math:`f_y`, :math:`f_z`, the energy variable is specified as
v_name, where name is the variable name.

Note that when the *energy* keyword is used during an energy
minimization, you must ensure that the formula defined for the
atom-style :doc:`variable <variable>` is consistent with the force
variable formulas (i.e., that :math:`-\vec\nabla E = \vec F`).
For example, if the force were a spring-like, :math:`\vec F = -k\vec x`, then
the energy formula should be :math:`E = \frac12 kx^2`.  If you do not do this
correctly, the minimization will not converge properly.

----------

.. 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>` *energy* option is supported by
this fix to add the potential energy inferred by the added force to
the global potential energy of the system as part of
:doc:`thermodynamic output <thermo_style>`.  The default setting for
this fix is :doc:`fix_modify energy no <fix_modify>`.  Note that this
energy is a fictitious quantity but is needed so that the
:doc:`minimize <minimize>` command can include the forces added by
this fix in a consistent manner (i.e., there is a decrease in
potential energy when atoms move in the direction of the added force).

The :doc:`fix_modify <fix_modify>` *virial* option is supported by
this fix to add the contribution due to the added forces on atoms to
both the global pressure and per-atom stress of the system via the
:doc:`compute pressure <compute_pressure>` and :doc:`compute
stress/atom <compute_stress_atom>` commands.  The former can be
accessed by :doc:`thermodynamic output <thermo_style>`.  The default
setting for this fix is :doc:`fix_modify virial no <fix_modify>`.

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 scalar and a global three-vector of forces,
which can be accessed by various :doc:`output commands
<Howto_output>`.  The scalar is the potential energy discussed above.
The vector is the total force on the group of atoms before the forces
on individual atoms are changed by the fix.  The scalar and 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 time step as
the iteration count during the minimization.

.. note::

   If you want the fictitious potential energy associated with the
   added forces to be included in the total potential energy of the
   system (the quantity being minimized), you MUST enable the
   :doc:`fix_modify <fix_modify>` *energy* option for this fix.

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

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

:doc:`fix setforce <fix_setforce>`, :doc:`fix aveforce <fix_aveforce>`

Default
"""""""

The option default for the every keyword is every = 1.
