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### fix external command

Syntax:

    fix ID group-ID external mode args

- ID, group-ID are documented in fix command
- external = style name of this fix command
- mode = pf/callback or pf/array

        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:

    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 library interface to modify certain LAMMPS properties on specific timesteps, similar to the way other fixes do. The external driver can be a C/C++ or Fortran program or a Python script.

If mode is pf/callback then the fix will make a callback every Ncall timesteps or minimization iteration 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 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:

    foo(void *ptr, bigint timestep, int nlocal, int *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.

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.

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 fix addforce command works.

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 run command, it could retrieve atom coordinates from LAMMPS, compute forces, set values in the fix external array, etc.

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

No information about this fix is written to binary restart files. None of the fix_modify options are relevant to this fix. No global or per-atom quantities are stored by this fix for access by various output commands. No parameter of this fix can be used with the start/stop keywords of the run command.

The forces due to this fix are imposed during an energy minimization, invoked by the minimize command. However, LAMMPS knows nothing about the energy associated with these forces. So you should perform the minimization based on a force tolerance, not an energy tolerance.

Restrictions: none

Related commands: none

Default: none