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<title>fix tune/kspace command — LAMMPS 15 May 2015 version documentation</title>
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<div class="section" id="fix-tune-kspace-command">
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<span id="index-0"></span><h1>fix tune/kspace command<a class="headerlink" href="#fix-tune-kspace-command" title="Permalink to this headline">¶</a></h1>
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<div class="section" id="syntax">
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<h2>Syntax<a class="headerlink" href="#syntax" title="Permalink to this headline">¶</a></h2>
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<div class="highlight-python"><div class="highlight"><pre>fix ID group-ID tune/kspace N
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</pre></div>
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</div>
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<ul class="simple">
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<li>ID, group-ID are documented in <a class="reference internal" href="fix.html"><em>fix</em></a> command</li>
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<li>tune/kspace = style name of this fix command</li>
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<li>N = invoke this fix every N steps</li>
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</ul>
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</div>
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<div class="section" id="examples">
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<h2>Examples<a class="headerlink" href="#examples" title="Permalink to this headline">¶</a></h2>
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<div class="highlight-python"><div class="highlight"><pre>fix 2 all tune/kspace 100
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</pre></div>
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</div>
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</div>
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<div class="section" id="description">
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<h2>Description<a class="headerlink" href="#description" title="Permalink to this headline">¶</a></h2>
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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.</p>
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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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number of particles).</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.</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.</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.</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 class="reference internal" href="kspace_style.html"><em>kspace_style</em></a> and <a class="reference internal" href="kspace_modify.html"><em>kspace_modify</em></a>
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commands. The prescribed accuracy will be maintained by this fix throughout
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the simulation.</p>
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<p>None of the <a class="reference internal" href="fix_modify.html"><em>fix_modify</em></a> options are relevant to this
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fix.</p>
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<p>No parameter of this fix can be used with the <em>start/stop</em> keywords of
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the <a class="reference internal" href="run.html"><em>run</em></a> command. This fix is not invoked during <a class="reference internal" href="minimize.html"><em>energy minimization</em></a>.</p>
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</div>
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<div class="section" id="restrictions">
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<h2>Restrictions<a class="headerlink" href="#restrictions" title="Permalink to this headline">¶</a></h2>
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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 class="reference internal" href="Section_start.html#start-3"><span>Making LAMMPS</span></a> section for more info.</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.</p>
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</div>
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<div class="section" id="related-commands">
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<h2>Related commands<a class="headerlink" href="#related-commands" title="Permalink to this headline">¶</a></h2>
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<p><a class="reference internal" href="kspace_style.html"><em>kspace_style</em></a>, <a class="reference internal" href="boundary.html"><em>boundary</em></a>
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<a class="reference internal" href="kspace_modify.html"><em>kspace_modify</em></a>, <a class="reference internal" href="pair_lj.html"><em>pair_style lj/cut/coul/long</em></a>, <a class="reference internal" href="pair_charmm.html"><em>pair_style lj/charmm/coul/long</em></a>, <a class="reference internal" href="pair_lj_long.html"><em>pair_style lj/long</em></a>, <a class="reference internal" href="pair_lj_long.html"><em>pair_style lj/long/coul/long</em></a>,
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<a class="reference internal" href="pair_buck.html"><em>pair_style buck/coul/long</em></a></p>
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</div>
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<div class="section" id="default">
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<h2>Default<a class="headerlink" href="#default" title="Permalink to this headline">¶</a></h2>
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