# fix tune/kspace command

## Syntax

    fix ID group-ID tune/kspace N

-   ID, group-ID are documented in [fix](fix) command
-   tune/kspace = style name of this fix command
-   N = invoke this fix every N steps

## Examples

``` LAMMPS
fix 2 all tune/kspace 100
```

## 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
Coulombic 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 heuristics 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) and [kspace_modify](kspace_modify)
commands. The prescribed accuracy will be maintained by this fix
throughout the simulation.

None of the [fix_modify](fix_modify) options are relevant to this fix.

No parameter of this fix can be used with the *start/stop* keywords of
the [run](run) command. This fix is not invoked during [energy
minimization](minimize).

## Restrictions

This fix is part of the KSPACE package. It is only enabled if LAMMPS was
built with that package. See the [Build package](Build_package) page for
more info.

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

This fix is not compatible with a hybrid pair style, long-range
dispersion, TIP4P water support, or long-range point dipole support.

## Related commands

[kspace_style](kspace_style), [boundary](boundary)
[kspace_modify](kspace_modify), [pair_style
lj/cut/coul/long](pair_lj_cut_coul), [pair_style
lj/charmm/coul/long](pair_charmm), [pair_style lj/long](pair_lj_long),
[pair_style lj/long/coul/long](pair_lj_long), [pair_style
buck/coul/long](pair_buck)

## Default