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doc/html/_sources/fix_tune_kspace.txt
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doc/html/_sources/fix_tune_kspace.txt
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.. index:: fix tune/kspace
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fix tune/kspace command
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=======================
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Syntax
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""""""
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.. parsed-literal::
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fix ID group-ID tune/kspace N
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* ID, group-ID are documented in :doc:`fix <fix>` command
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* tune/kspace = style name of this fix command
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* N = invoke this fix every N steps
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Examples
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""""""""
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.. parsed-literal::
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fix 2 all tune/kspace 100
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Description
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"""""""""""
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This fix tests each kspace style (Ewald, PPPM, and MSM), and
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automatically selects the fastest style to use for the remainder
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of the run. If the fastest style is Ewald or PPPM, the fix also
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adjusts the coulomb cutoff towards optimal speed. Future versions
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of this fix will automatically select other kspace parameters
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to use for maximum simulation speed. The kspace parameters may
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include the style, cutoff, grid points in each direction, order,
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Ewald parameter, MSM parallelization cut-point, MPI tasks to use, etc.
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The rationale for this fix is to provide the user with
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as-fast-as-possible simulations that include long-range electrostatics
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(kspace) while meeting the user-prescribed accuracy requirement. A
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simple heuristic could never capture the optimal combination of
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parameters for every possible run-time scenario. But by performing
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short tests of various kspace parameter sets, this fix allows
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parameters to be tailored specifically to the user's machine, MPI
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ranks, use of threading or accelerators, the simulated system, and the
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simulation details. In addition, it is possible that parameters could
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be evolved with the simulation on-the-fly, which is useful for systems
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that are dynamically evolving (e.g. changes in box size/shape or
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number of particles).
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When this fix is invoked, LAMMPS will perform short timed tests of
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various parameter sets to determine the optimal parameters. Tests are
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performed on-the-fly, with a new test initialized every N steps. N should
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be chosen large enough so that adequate CPU time lapses between tests,
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thereby providing statistically significant timings. But N should not be
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chosen to be so large that an unfortunate parameter set test takes an
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inordinate amount of wall time to complete. An N of 100 for most problems
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seems reasonable. Once an optimal parameter set is found, that set is
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used for the remainder of the run.
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This fix uses heristics to guide it's selection of parameter sets to test,
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but the actual timed results will be used to decide which set to use in the
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simulation.
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It is not necessary to discard trajectories produced using sub-optimal
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parameter sets, or a mix of various parameter sets, since the user-prescribed
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accuracy will have been maintained throughout. However, some users may prefer
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to use this fix only to discover the optimal parameter set for a given setup
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that can then be used on subsequent production runs.
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This fix starts with kspace parameters that are set by the user with the
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:doc:`kspace_style <kspace_style>` and :doc:`kspace_modify <kspace_modify>`
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commands. The prescribed accuracy will be maintained by this fix throughout
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the simulation.
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None of the :doc:`fix_modify <fix_modify>` options are relevant to this
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fix.
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No parameter of this fix can be used with the *start/stop* keywords of
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the :doc:`run <run>` command. This fix is not invoked during :doc:`energy minimization <minimize>`.
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Restrictions
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""""""""""""
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This fix is part of the KSPACE package. It is only enabled if LAMMPS was
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built with that package. See the :ref:`Making LAMMPS <start_3>` section for more info.
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Do not set "neigh_modify once yes" or else this fix will never be
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called. Reneighboring is required.
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Related commands
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""""""""""""""""
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:doc:`kspace_style <kspace_style>`, :doc:`boundary <boundary>`
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:doc:`kspace_modify <kspace_modify>`, :doc:`pair_style lj/cut/coul/long <pair_lj>`, :doc:`pair_style lj/charmm/coul/long <pair_charmm>`, :doc:`pair_style lj/long <pair_lj_long>`, :doc:`pair_style lj/long/coul/long <pair_lj_long>`,
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:doc:`pair_style buck/coul/long <pair_buck>`
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Default
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"""""""
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----------
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.. _lws: http://lammps.sandia.gov
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.. _ld: Manual.html
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.. _lc: Section_commands.html#comm
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