.. index:: fix external

fix external command
====================

Syntax
""""""

.. code-block:: LAMMPS

   fix ID group-ID external mode args

* ID, group-ID are documented in :doc:`fix <fix>` command
* external = style name of this fix command
* mode = *pf/callback* or *pf/array*

  .. parsed-literal::

       *pf/callback* args = Ncall Napply
         Ncall = make callback every Ncall steps
         Napply = apply callback forces every Napply steps
       *pf/array* args = Napply
         Napply = apply array forces every Napply steps

Examples
""""""""

.. code-block:: LAMMPS

   fix 1 all external pf/callback 1 1
   fix 1 all external pf/callback 100 1
   fix 1 all external pf/array 10

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

This fix allows external programs that are running LAMMPS through its
:doc:`library interface <Howto_library>` to modify certain LAMMPS
properties on specific timesteps, similar to the way other fixes do.
The external driver can be a :doc:`C/C++ or Fortran program <Howto_library>` or a :doc:`Python script <Python_head>`.

----------

If mode is *pf/callback* then the fix will make a callback every
*Ncall* timesteps or minimization iterations to the external program.
The external program computes forces on atoms by setting values in an
array owned by the fix.  The fix then adds these forces to each atom
in the group, once every *Napply* steps, similar to the way the :doc:`fix addforce <fix_addforce>` command works.  Note that if *Ncall* >
*Napply*, the force values produced by one callback will persist, and
be used multiple times to update atom forces.

The callback function "foo" is invoked by the fix as:

.. code-block:: c++

   foo(void *ptr, bigint timestep, int nlocal, tagint *ids, double **x, double **fexternal);

The arguments are as follows:

* *ptr* = pointer provided by and simply passed back to external driver
* *timestep* = current LAMMPS timestep
* *nlocal* = # of atoms on this processor
* *ids* = list of atom IDs on this processor
* *x* = coordinates of atoms on this processor
* *fexternal* = forces to add to atoms on this processor

Note that *timestep* is a "bigint" which is defined in src/lmptype.h,
typically as a 64-bit integer. And *ids* is a pointer to type "tagint"
which is typically a 32-bit integer unless LAMMPS is compiled with
-DLAMMPS_BIGBIG. For more info please see the :ref:`build settings
<size>` section of the manual.  Finally, *fexternal* are the forces
returned by the driver program.

The fix has a set_callback() method which the external driver can call
to pass a pointer to its foo() function.  See the
couple/lammps_quest/lmpqst.cpp file in the LAMMPS distribution for an
example of how this is done.  This sample application performs
classical MD using quantum forces computed by a density functional
code `Quest <quest_>`_.

.. _quest: https://dft.sandia.gov/Quest

----------

If mode is *pf/array* then the fix simply stores force values in an
array.  The fix adds these forces to each atom in the group, once
every *Napply* steps, similar to the way the :doc:`fix addforce
<fix_addforce>` command works.

The name of the public force array provided by the FixExternal
class is

.. code-block:: c++

   double **fexternal;

It is allocated by the FixExternal class as an (N,3) array where N is
the number of atoms owned by a processor.  The 3 corresponds to the
fx, fy, fz components of force.

It is up to the external program to set the values in this array to
the desired quantities, as often as desired.  For example, the driver
program might perform an MD run in stages of 1000 timesteps each.  In
between calls to the LAMMPS :doc:`run <run>` command, it could retrieve
atom coordinates from LAMMPS, compute forces, set values in fexternal,
etc.

----------

To use this fix during energy minimization, the energy corresponding
to the added forces must also be set so as to be consistent with the
added forces.  Otherwise the minimization will not converge correctly.
Correspondingly, the global virial needs to be updated to be use this
fix with variable cell calculations (e.g. :doc:`fix box/relax <fix_box_relax>`
or :doc:`fix npt <fix_nh>`).

This can be done from the external driver by calling these public
methods of the FixExternal class:

.. code-block:: c++

   void set_energy_global(double eng);
   void set_virial_global(double *virial);

where *eng* is the potential energy, and *virial* an array of the 6
stress tensor components.  Eng is an extensive quantity,
meaning it should be the sum over per-atom energies of all affected
atoms.  It should also be provided in :doc:`energy units <units>`
consistent with the simulation.  See the details below for how to
ensure this energy setting is used appropriately in a minimization.

Additional public methods that the caller can use to update system
properties are:

.. code-block:: c++

   void set_energy_peratom(double *eng);
   void set_virial_peratom(double **virial);
   void set_vector_length(int n);
   void set_vector(int idx, double val);

These enable setting per-atom energy and  per-atom stress contributions,
the length and individual values of a global vector of properties that
the caller code may want to communicate  to LAMMPS
(e.g. for use in :doc:`fix ave/time <fix_ave_time>` or in
:doc:`equal-style variables <variable>` or for
:doc:`custom thermo output <thermo_style>`.

----------

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 set by the external driver to
both the global potential energy and peratom potential energies of the
system as part of :doc:`thermodynamic output <thermo_style>` or output
by the :doc:`compute pe/atom <compute_pe_atom>` command.  The default
setting for this fix is :doc:`fix_modify energy yes <fix_modify>`.
Note that this energy may be a fictitious quantity but it 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 computed by the external program 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 yes <fix_modify>`.

This fix computes a global scalar, a global vector, and a per-atom array
which can be accessed by various :doc:`output commands <Howto_output>`.
The scalar is the potential energy discussed above.  The scalar stored
by this fix is "extensive".
The global vector has a custom length and needs to be set by the external
program using the
:cpp:func:`lammps_fix_external_set_vector() <lammps_fix_external_set_vector>`
and :cpp:func:`lammps_fix_external_set_vector_length()
<lammps_fix_external_set_vector_length>`
calls of the LAMMPS library interface or the equivalent call of the Python
or Fortran modules.
The per-atom array has 3 values for each atom and is the applied external
force.

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.

.. 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 not disable the
   :doc:`fix_modify <fix_modify>` *energy* option for this fix.

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

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

none


Default
"""""""

none
