forked from lijiext/lammps
114 lines
4.0 KiB
Plaintext
114 lines
4.0 KiB
Plaintext
"LAMMPS WWW Site"_lws - "LAMMPS Documentation"_ld - "LAMMPS Commands"_lc :c
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:link(lws,http://lammps.sandia.gov)
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:link(ld,Manual.html)
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:link(lc,Section_commands.html#comm)
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:line
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fix external command :h3
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[Syntax:]
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fix ID group-ID external mode args :pre
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ID, group-ID are documented in "fix"_fix.html command :ulb,l
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external = style name of this fix command :l
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mode = {pf/callback} or {pf/array} :l
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{pf/callback} args = Ncall Napply
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Ncall = make callback every Ncall steps
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Napply = apply callback forces every Napply steps
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{pf/array} args = Napply
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Napply = apply array forces every Napply steps :pre
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:ule
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[Examples:]
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fix 1 all external pf/callback 1 1
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fix 1 all external pf/callback 100 1
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fix 1 all external pf/array 10 :pre
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[Description:]
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This fix allows external programs that are running LAMMPS through its
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"library interface"_Section_howto.html#howto_19 to modify certain
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LAMMPS properties on specific timesteps, similar to the way other
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fixes do. The external driver can be a "C/C++ or Fortran
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program"_Section_howto.html#howto_19 or a "Python
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script"_Section_python.html.
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:line
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If mode is {pf/callback} then the fix will make a callback every
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{Ncall} timesteps or minimization iteration to the external program.
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The external program computes forces on atoms by setting values in an
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array owned by the fix. The fix then adds these forces to each atom
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in the group, once every {Napply} steps, similar to the way the "fix
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addforce"_fix_addforce.html command works. Note that if {Ncall} >
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{Napply}, the force values produced by one callback will persist, and
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be used multiple times to update atom forces.
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The callback function "foo" is invoked by the fix as:
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foo(void *ptr, bigint timestep, int nlocal, int *ids, double **x, double **fexternal); :pre
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The arguments are as follows:
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ptr = pointer provided by and simply passed back to external driver
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timestep = current LAMMPS timestep
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nlocal = # of atoms on this processor
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ids = list of atom IDs on this processor
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x = coordinates of atoms on this processor
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fexternal = forces to add to atoms on this processor :ul
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Note that timestep is a "bigint" which is defined in src/lmptype.h,
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typically as a 64-bit integer.
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Fexternal are the forces returned by the driver program.
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The fix has a set_callback() method which the external driver can call
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to pass a pointer to its foo() function. See the
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couple/lammps_quest/lmpqst.cpp file in the LAMMPS distribution for an
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example of how this is done. This sample application performs
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classical MD using quantum forces computed by a density functional
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code "Quest"_quest.
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:link(quest,http://dft.sandia.gov/Quest)
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:line
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If mode is {pf/array} then the fix simply stores force values in an
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array. The fix adds these forces to each atom in the group, once
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every {Napply} steps, similar to the way the "fix
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addforce"_fix_addforce.html command works.
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It is up to the external program to set the values in this array to
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the desired quantities, as often as desired. For example, the driver
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program might perform an MD run in stages of 1000 timesteps each. In
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between calls to the LAMMPS "run"_run.html command, it could retrieve
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atom coordinates from LAMMPS, compute forces, set values in the fix
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external array, etc.
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:line
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[Restart, fix_modify, output, run start/stop, minimize info:]
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No information about this fix is written to "binary restart
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files"_restart.html. None of the "fix_modify"_fix_modify.html options
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are relevant to this fix. No global or per-atom quantities are stored
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by this fix for access by various "output
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commands"_Section_howto.html#howto_15. No parameter of this fix can
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be used with the {start/stop} keywords of the "run"_run.html command.
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The forces due to this fix are imposed during an energy minimization,
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invoked by the "minimize"_minimize.html command. However, LAMMPS
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knows nothing about the energy associated with these forces. So you
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should perform the minimization based on a force tolerance, not an
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energy tolerance.
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[Restrictions:] none
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[Related commands:] none
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[Default:] none
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