2013-06-11 07:43:19 +08:00
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<CENTER><A HREF = "http://lammps.sandia.gov">LAMMPS WWW Site</A> - <A HREF = "Manual.html">LAMMPS Documentation</A> - <A HREF = "Section_commands.html#comm">LAMMPS Commands</A>
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<H3>fix tune/kspace command
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</H3>
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<P><B>Syntax:</B>
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</P>
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<PRE>fix ID group-ID tune/kspace N
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</PRE>
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<UL><LI>ID, group-ID are documented in <A HREF = "fix.html">fix</A> command
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<LI>tune/kspace = style name of this fix command
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<LI>N = invoke this fix every N steps
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</UL>
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<P><B>Examples:</B>
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</P>
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<PRE>fix 2 all tune/kspace 100
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</PRE>
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<P><B>Description:</B>
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</P>
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<P>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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</P>
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2013-06-28 07:24:46 +08:00
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<P>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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2013-06-11 07:43:19 +08:00
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number of particles).
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</P>
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<P>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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</P>
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<P>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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</P>
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<P>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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</P>
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<P>This fix starts with kspace parameters that are set by the user with the
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<A HREF = "kspace_style.html">kspace_style</A> and <A HREF = "kspace_modify.html">kspace_modify</A>
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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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</P>
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<P>None of the <A HREF = "fix_modify.html">fix_modify</A> options are relevant to this
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fix.
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</P>
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<P>No parameter of this fix can be used with the <I>start/stop</I> keywords of
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the <A HREF = "run.html">run</A> command. This fix is not invoked during <A HREF = "minimize.html">energy
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minimization</A>.
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</P>
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<P><B>Restrictions:</B>
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</P>
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<P>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 <A HREF = "Section_start.html#start_3">Making
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LAMMPS</A> section for more info.
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</P>
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<P>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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</P>
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<P><B>Related commands:</B>
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</P>
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<P><A HREF = "kspace_style.html">kspace_style</A>, <A HREF = "boundary.html">boundary</A>
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<A HREF = "kspace_modify.html">kspace_modify</A>, <A HREF = "pair_lj.html">pair_style
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lj/cut/coul/long</A>, <A HREF = "pair_charmm.html">pair_style
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2013-07-23 23:21:46 +08:00
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lj/charmm/coul/long</A>, <A HREF = "pair_lj_long.html">pair_style
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lj/long</A>, <A HREF = "pair_lj_long.html">pair_style
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lj/long/coul/long</A>,
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<A HREF = "pair_buck.html">pair_style buck/coul/long</A>
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2013-06-11 07:43:19 +08:00
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</P>
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<P><B>Default:</B>
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</P>
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<HR>
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</HTML>
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