"LAMMPS WWW Site"_lws - "LAMMPS Documentation"_ld - "LAMMPS Commands"_lc :c

:link(lws,http://lammps.sandia.gov)
:link(ld,Manual.html)
:link(lc,Section_commands.html#comm)

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fix tune/kspace command :h3

[Syntax:]

fix ID group-ID tune/kspace N :pre

ID, group-ID are documented in "fix"_fix.html command :ulb,l
tune/kspace = style name of this fix command :l
N = invoke this fix every N steps :l
:ule

[Examples:]

fix 2 all tune/kspace 100 :pre

[Description:]

This fix tests each kspace style (Ewald, PPPM, and MSM), and 
automatically selects the fastest style to use for the remainder
of the run. If the fastest style is Ewald or PPPM, the fix also
adjusts the coulomb cutoff towards optimal speed. Future versions 
of this fix will automatically select other kspace parameters 
to use for maximum simulation speed. The kspace parameters may 
include the style, cutoff, grid points in each direction, order, 
Ewald parameter, MSM parallelization cut-point, MPI tasks to use, etc.

The rationale for this fix is to provide the user with
as-fast-as-possible simulations that include long-range electrostatics
(kspace) while meeting the user-prescribed accuracy requirement. A
simple heuristic could never capture the optimal combination of
parameters for every possible run-time scenario. But by performing
short tests of various kspace parameter sets, this fix allows
parameters to be tailored specifically to the user's machine, MPI
ranks, use of threading or accelerators, the simulated system, and the
simulation details. In addition, it is possible that parameters could
be evolved with the simulation on-the-fly, which is useful for systems
that are dynamically evolving (e.g. changes in box size/shape or
number of particles).

When this fix is invoked, LAMMPS will perform short timed tests of 
various parameter sets to determine the optimal parameters. Tests are 
performed on-the-fly, with a new test initialized every N steps. N should
be chosen large enough so that adequate CPU time lapses between tests, 
thereby providing statistically significant timings. But N should not be
chosen to be so large that an unfortunate parameter set test takes an 
inordinate amount of wall time to complete. An N of 100 for most problems
seems reasonable. Once an optimal parameter set is found, that set is 
used for the remainder of the run.

This fix uses heristics to guide it's selection of parameter sets to test,
but the actual timed results will be used to decide which set to use in the
simulation.

It is not necessary to discard trajectories produced using sub-optimal 
parameter sets, or a mix of various parameter sets, since the user-prescribed
accuracy will have been maintained throughout. However, some users may prefer
to use this fix only to discover the optimal parameter set for a given setup 
that can then be used on subsequent production runs.

This fix starts with kspace parameters that are set by the user with the
"kspace_style"_kspace_style.html and "kspace_modify"_kspace_modify.html 
commands. The prescribed accuracy will be maintained by this fix throughout 
the simulation.

None of the "fix_modify"_fix_modify.html options are relevant to this
fix.

No parameter of this fix can be used with the {start/stop} keywords of
the "run"_run.html command.  This fix is not invoked during "energy
minimization"_minimize.html.

[Restrictions:]

This fix is part of the KSPACE package.  It is only enabled if LAMMPS was
built with that package.  See the "Making
LAMMPS"_Section_start.html#start_3 section for more info.

Do not set "neigh_modify once yes" or else this fix will never be
called.  Reneighboring is required.

[Related commands:]

"kspace_style"_kspace_style.html, "boundary"_boundary.html
"kspace_modify"_kspace_modify.html, "pair_style
lj/cut/coul/long"_pair_lj.html, "pair_style
lj/charmm/coul/long"_pair_charmm.html, "pair_style
lj/long"_pair_lj_long.html, "pair_style
lj/long/coul/long"_pair_lj_long.html,
"pair_style buck/coul/long"_pair_buck.html

[Default:]

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