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<li class="toctree-l1"><a class="reference internal" href="Section_intro.html">1. Introduction</a></li>
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<li class="toctree-l1"><a class="reference internal" href="Section_start.html">2. Getting Started</a></li>
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<li class="toctree-l1"><a class="reference internal" href="Section_commands.html">3. Commands</a></li>
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<li class="toctree-l1"><a class="reference internal" href="Section_accelerate.html">5. Accelerating LAMMPS performance</a></li>
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<li class="toctree-l1"><a class="reference internal" href="Section_howto.html">6. How-to discussions</a></li>
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<li class="toctree-l1"><a class="reference internal" href="Section_example.html">7. Example problems</a></li>
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<li class="toctree-l1"><a class="reference internal" href="Section_perf.html">8. Performance & scalability</a></li>
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<li class="toctree-l1"><a class="reference internal" href="Section_errors.html">12. Errors</a></li>
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<li>compute chunk/atom command</li>
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<a href="Section_commands.html#comm">Commands</a>
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<div class="section" id="compute-chunk-atom-command">
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<span id="index-0"></span><h1>compute chunk/atom command</h1>
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<div class="section" id="syntax">
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<h2>Syntax</h2>
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<div class="highlight-default"><div class="highlight"><pre><span></span><span class="n">compute</span> <span class="n">ID</span> <span class="n">group</span><span class="o">-</span><span class="n">ID</span> <span class="n">chunk</span><span class="o">/</span><span class="n">atom</span> <span class="n">style</span> <span class="n">args</span> <span class="n">keyword</span> <span class="n">values</span> <span class="o">...</span>
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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="compute.html"><span class="doc">compute</span></a> command</li>
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<li>chunk/atom = style name of this compute command</li>
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</ul>
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<pre class="literal-block">
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style = <em>bin/1d</em> or <em>bin/2d</em> or <em>bin/3d</em> or <em>bin/sphere</em> or <em>type</em> or <em>molecule</em> or <em>compute/fix/variable</em>
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<em>bin/1d</em> args = dim origin delta
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dim = <em>x</em> or <em>y</em> or <em>z</em>
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origin = <em>lower</em> or <em>center</em> or <em>upper</em> or coordinate value (distance units)
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delta = thickness of spatial bins in dim (distance units)
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<em>bin/2d</em> args = dim origin delta dim origin delta
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dim = <em>x</em> or <em>y</em> or <em>z</em>
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origin = <em>lower</em> or <em>center</em> or <em>upper</em> or coordinate value (distance units)
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delta = thickness of spatial bins in dim (distance units)
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<em>bin/3d</em> args = dim origin delta dim origin delta dim origin delta
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dim = <em>x</em> or <em>y</em> or <em>z</em>
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origin = <em>lower</em> or <em>center</em> or <em>upper</em> or coordinate value (distance units)
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delta = thickness of spatial bins in dim (distance units)
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<em>bin/sphere</em> args = xorig yorig zorig rmin rmax nsbin
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xorig,yorig,zorig = center point of sphere
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srmin,srmax = bin from sphere radius rmin to rmax
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nsbin = # of spherical shell bins between rmin and rmax
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<em>bin/cylinder</em> args = dim origin delta c1 c2 rmin rmax ncbin
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dim = <em>x</em> or <em>y</em> or <em>z</em> = axis of cylinder axis
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origin = <em>lower</em> or <em>center</em> or <em>upper</em> or coordinate value (distance units)
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delta = thickness of spatial bins in dim (distance units)
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c1,c2 = coords of cylinder axis in other 2 dimensions (distance units)
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crmin,crmax = bin from cylinder radius rmin to rmax (distance units)
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ncbin = # of concentric circle bins between rmin and rmax
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<em>type</em> args = none
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<em>molecule</em> args = none
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<em>compute/fix/variable</em> = c_ID, c_ID[I], f_ID, f_ID[I], v_name with no args
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c_ID = per-atom vector calculated by a compute with ID
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c_ID[I] = Ith column of per-atom array calculated by a compute with ID
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f_ID = per-atom vector calculated by a fix with ID
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f_ID[I] = Ith column of per-atom array calculated by a fix with ID
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v_name = per-atom vector calculated by an atom-style variable with name
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</pre>
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<ul class="simple">
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<li>zero or more keyword/values pairs may be appended</li>
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<li>keyword = <em>region</em> or <em>nchunk</em> or <em>static</em> or <em>compress</em> or <em>bound</em> or <em>discard</em> or <em>pbc</em> or <em>units</em></li>
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</ul>
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<pre class="literal-block">
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<em>region</em> value = region-ID
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region-ID = ID of region atoms must be in to be part of a chunk
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<em>nchunk</em> value = <em>once</em> or <em>every</em>
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once = only compute the number of chunks once
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every = re-compute the number of chunks whenever invoked
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<em>limit</em> values = 0 or Nc max or Nc exact
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0 = no limit on the number of chunks
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Nc max = limit number of chunks to be <= Nc
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Nc exact = set number of chunks to exactly Nc
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<em>ids</em> value = <em>once</em> or <em>nfreq</em> or <em>every</em>
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once = assign chunk IDs to atoms only once, they persist thereafter
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nfreq = assign chunk IDs to atoms only once every Nfreq steps (if invoked by <a class="reference internal" href="fix_ave_chunk.html"><span class="doc">fix ave/chunk</span></a> which sets Nfreq)
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every = assign chunk IDs to atoms whenever invoked
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<em>compress</em> value = <em>yes</em> or <em>no</em>
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yes = compress chunk IDs to eliminate IDs with no atoms
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no = do not compress chunk IDs even if some IDs have no atoms
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<em>discard</em> value = <em>yes</em> or <em>no</em> or <em>mixed</em>
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yes = discard atoms with out-of-range chunk IDs by assigning a chunk ID = 0
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no = keep atoms with out-of-range chunk IDs by assigning a valid chunk ID
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mixed = keep or discard such atoms according to spatial binning rule
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<em>bound</em> values = x/y/z lo hi
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x/y/z = <em>x</em> or <em>y</em> or <em>z</em> to bound sptial bins in this dimension
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lo = <em>lower</em> or coordinate value (distance units)
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hi = <em>upper</em> or coordinate value (distance units)
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<em>pbc</em> value = <em>no</em> or <em>yes</em>
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yes = use periodic distance for bin/sphere and bin/cylinder styles
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<em>units</em> value = <em>box</em> or <em>lattice</em> or <em>reduced</em>
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</pre>
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</div>
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<div class="section" id="examples">
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<h2>Examples</h2>
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<div class="highlight-default"><div class="highlight"><pre><span></span><span class="n">compute</span> <span class="mi">1</span> <span class="nb">all</span> <span class="n">chunk</span><span class="o">/</span><span class="n">atom</span> <span class="nb">type</span>
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<span class="n">compute</span> <span class="mi">1</span> <span class="nb">all</span> <span class="n">chunk</span><span class="o">/</span><span class="n">atom</span> <span class="nb">bin</span><span class="o">/</span><span class="mi">1</span><span class="n">d</span> <span class="n">z</span> <span class="n">lower</span> <span class="mf">0.02</span> <span class="n">units</span> <span class="n">reduced</span>
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<span class="n">compute</span> <span class="mi">1</span> <span class="nb">all</span> <span class="n">chunk</span><span class="o">/</span><span class="n">atom</span> <span class="nb">bin</span><span class="o">/</span><span class="mi">2</span><span class="n">d</span> <span class="n">z</span> <span class="n">lower</span> <span class="mf">1.0</span> <span class="n">y</span> <span class="mf">0.0</span> <span class="mf">2.5</span>
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<span class="n">compute</span> <span class="mi">1</span> <span class="nb">all</span> <span class="n">chunk</span><span class="o">/</span><span class="n">atom</span> <span class="n">molecule</span> <span class="n">region</span> <span class="n">sphere</span> <span class="n">nchunk</span> <span class="n">once</span> <span class="n">ids</span> <span class="n">once</span> <span class="n">compress</span> <span class="n">yes</span>
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<span class="n">compute</span> <span class="mi">1</span> <span class="nb">all</span> <span class="n">chunk</span><span class="o">/</span><span class="n">atom</span> <span class="nb">bin</span><span class="o">/</span><span class="n">sphere</span> <span class="mi">5</span> <span class="mi">5</span> <span class="mi">5</span> <span class="mf">2.0</span> <span class="mf">5.0</span> <span class="mi">5</span> <span class="n">discard</span> <span class="n">yes</span>
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<span class="n">compute</span> <span class="mi">1</span> <span class="nb">all</span> <span class="n">chunk</span><span class="o">/</span><span class="n">atom</span> <span class="nb">bin</span><span class="o">/</span><span class="n">cylinder</span> <span class="n">z</span> <span class="n">lower</span> <span class="mi">2</span> <span class="mi">10</span> <span class="mi">10</span> <span class="mf">2.0</span> <span class="mf">5.0</span> <span class="mi">3</span> <span class="n">discard</span> <span class="n">yes</span>
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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</h2>
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<p>Define a computation that calculates an integer chunk ID from 1 to
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Nchunk for each atom in the group. Values of chunk IDs are determined
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by the <em>style</em> of chunk, which can be based on atom type or molecule
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ID or spatial binning or a per-atom property or value calculated by
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another <a class="reference internal" href="compute.html"><span class="doc">compute</span></a>, <a class="reference internal" href="fix.html"><span class="doc">fix</span></a>, or <a class="reference internal" href="variable.html"><span class="doc">atom-style variable</span></a>. Per-atom chunk IDs can be used by other
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computes with “chunk” in their style name, such as <a class="reference internal" href="compute_com_chunk.html"><span class="doc">compute com/chunk</span></a> or <a class="reference internal" href="compute_msd_chunk.html"><span class="doc">compute msd/chunk</span></a>. Or they can be used by the <a class="reference internal" href="fix_ave_chunk.html"><span class="doc">fix ave/chunk</span></a> command to sum and time average a
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variety of per-atom properties over the atoms in each chunk. Or they
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can simply be accessed by any command that uses per-atom values from a
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compute as input, as discussed in <a class="reference internal" href="Section_howto.html#howto-15"><span class="std std-ref">Section_howto 15</span></a>.</p>
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<p>See <a class="reference internal" href="Section_howto.html#howto-23"><span class="std std-ref">Section_howto 23</span></a> for an overview of
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how this compute can be used with a variety of other commands to
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tabulate properties of a simulation. The howto section gives several
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examples of input script commands that can be used to calculate
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interesting properties.</p>
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<p>Conceptually it is important to realize that this compute does two
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simple things. First, it sets the value of <em>Nchunk</em> = the number of
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chunks, which can be a constant value or change over time. Second, it
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assigns each atom to a chunk via a chunk ID. Chunk IDs range from 1
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to <em>Nchunk</em> inclusive; some chunks may have no atoms assigned to them.
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Atoms that do not belong to any chunk are assigned a value of 0. Note
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that the two operations are not always performed together. For
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example, spatial bins can be setup once (which sets <em>Nchunk</em>), and
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atoms assigned to those bins many times thereafter (setting their
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chunk IDs).</p>
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<p>All other commands in LAMMPS that use chunk IDs assume there are
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<em>Nchunk</em> number of chunks, and that every atom is assigned to one of
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those chunks, or not assigned to any chunk.</p>
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<p>There are many options for specifying for how and when <em>Nchunk</em> is
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calculated, and how and when chunk IDs are assigned to atoms. The
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details depend on the chunk <em>style</em> and its <em>args</em>, as well as
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optional keyword settings. They can also depend on whether a <a class="reference internal" href="fix_ave_chunk.html"><span class="doc">fix ave/chunk</span></a> command is using this compute, since
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that command requires <em>Nchunk</em> to remain static across windows of
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timesteps it specifies, while it accumulates per-chunk averages.</p>
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<p>The details are described below.</p>
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<p>The different chunk styles operate as follows. For each style, how it
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calculates <em>Nchunk</em> and assigns chunk IDs to atoms is explained. Note
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that using the optional keywords can change both of those actions, as
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described further below where the keywords are discussed.</p>
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<hr class="docutils" />
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<p>The <em>binning</em> styles perform a spatial binning of atoms, and assign an
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atom the chunk ID corresponding to the bin number it is in. <em>Nchunk</em>
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is set to the number of bins, which can change if the simulation box
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size changes.</p>
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<p>The <em>bin/1d</em>, <em>bin/2d</em>, and <em>bin/3d</em> styles define bins as 1d layers
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(slabs), 2d pencils, or 3d boxes. The <em>dim</em>, <em>origin</em>, and <em>delta</em>
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settings are specified 1, 2, or 3 times. For 2d or 3d bins, there is
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no restriction on specifying dim = x before dim = y or z, or dim = y
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before dim = z. Bins in a particular <em>dim</em> have a bin size in that
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dimension given by <em>delta</em>. In each dimension, bins are defined
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relative to a specified <em>origin</em>, which may be the lower/upper edge of
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the simulation box (in that dimension), or its center point, or a
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specified coordinate value. Starting at the origin, sufficient bins
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are created in both directions to completely span the simulation box
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or the bounds specified by the optional <em>bounds</em> keyword.</p>
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<p>For orthogonal simulation boxes, the bins are layers, pencils, or
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boxes aligned with the xyz coordinate axes. For triclinic
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(non-orthogonal) simulation boxes, the bin faces are parallel to the
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tilted faces of the simulation box. See <a class="reference internal" href="Section_howto.html#howto-12"><span class="std std-ref">this section</span></a> of the manual for a discussion of
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the geometry of triclinic boxes in LAMMPS. As described there, a
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tilted simulation box has edge vectors a,b,c. In that nomenclature,
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bins in the x dimension have faces with normals in the “b” cross “c”
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direction. Bins in y have faces normal to the “a” cross “c”
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direction. And bins in z have faces normal to the “a” cross “b”
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direction. Note that in order to define the size and position of
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these bins in an unambiguous fashion, the <em>units</em> option must be set
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to <em>reduced</em> when using a triclinic simulation box, as noted below.</p>
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<p>The meaning of <em>origin</em> and <em>delta</em> for triclinic boxes is as follows.
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Consider a triclinic box with bins that are 1d layers or slabs in the
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x dimension. No matter how the box is tilted, an <em>origin</em> of 0.0
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means start layers at the lower “b” cross “c” plane of the simulation
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box and an <em>origin</em> of 1.0 means to start layers at the upper “b”
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cross “c” face of the box. A <em>delta</em> value of 0.1 in <em>reduced</em> units
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means there will be 10 layers from 0.0 to 1.0, regardless of the
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current size or shape of the simulation box.</p>
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<p>The <em>bin/sphere</em> style defines a set of spherical shell bins around
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the origin (<em>xorig</em>,<em>yorig</em>,<em>zorig</em>), using <em>nsbin</em> bins with radii
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equally spaced between <em>srmin</em> and <em>srmax</em>. This is effectively a 1d
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vector of bins. For example, if <em>srmin</em> = 1.0 and <em>srmax</em> = 10.0 and
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<em>nsbin</em> = 9, then the first bin spans 1.0 < r < 2.0, and the last bin
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spans 9.0 < r 10.0. The geometry of the bins is the same whether the
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simulation box is orthogonal or triclinic; i.e. the spherical shells
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are not tilted or scaled differently in different dimensions to
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transform them into ellipsoidal shells.</p>
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<p>The <em>bin/cylinder</em> style defines bins for a cylinder oriented along
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the axis <em>dim</em> with the axis coordinates in the other two radial
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dimensions at (<em>c1</em>,<em>c2</em>). For dim = x, c1/c2 = y/z; for dim = y,
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c1/c2 = x/z; for dim = z, c1/c2 = x/y. This is effectively a 2d array
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of bins. The first dimension is along the cylinder axis, the second
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dimension is radially outward from the cylinder axis. The bin size
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and positions along the cylinder axis are specified by the <em>origin</em>
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and <em>delta</em> values, the same as for the <em>bin/1d</em>, <em>bin/2d</em>, and
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<em>bin/3d</em> styles. There are <em>ncbin</em> concentric circle bins in the
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radial direction from the cylinder axis with radii equally spaced
|
|
between <em>crmin</em> and <em>crmax</em>. For example, if <em>crmin</em> = 1.0 and
|
|
<em>crmax</em> = 10.0 and <em>ncbin</em> = 9, then the first bin spans 1.0 < r <
|
|
2.0, and the last bin spans 9.0 < r 10.0. The geometry of the bins in
|
|
the radial dimensions is the same whether the simulation box is
|
|
orthogonal or triclinic; i.e. the concetric circles are not tilted or
|
|
scaled differently in the two different dimensions to transform them
|
|
into ellipses.</p>
|
|
<p>The created bins (and hence the chunk IDs) are numbered consecutively
|
|
from 1 to the number of bins = <em>Nchunk</em>. For <em>bin2d</em> and <em>bin3d</em>, the
|
|
numbering varies most rapidly in the first dimension (which could be
|
|
x, y, or z), next rapidly in the 2nd dimension, and most slowly in the
|
|
3rd dimension. For <em>bin/sphere</em>, the bin with smallest radii is chunk
|
|
1 and the bni with largest radii is chunk Nchunk = <em>ncbin</em>. For
|
|
<em>bin/cylinder</em>, the numbering varies most rapidly in the dimension
|
|
along the cylinder axis and most slowly in the radial direction.</p>
|
|
<p>Each time this compute is invoked, each atom is mapped to a bin based
|
|
on its current position. Note that between reneighboring timesteps,
|
|
atoms can move outside the current simulation box. If the box is
|
|
periodic (in that dimension) the atom is remapping into the periodic
|
|
box for purposes of binning. If the box in not periodic, the atom may
|
|
have moved outside the bounds of all bins. If an atom is not inside
|
|
any bin, the <em>discard</em> keyword is used to determine how a chunk ID is
|
|
assigned to the atom.</p>
|
|
<hr class="docutils" />
|
|
<p>The <em>type</em> style uses the atom type as the chunk ID. <em>Nchunk</em> is set
|
|
to the number of atom types defined for the simulation, e.g. via the
|
|
<a class="reference internal" href="create_box.html"><span class="doc">create_box</span></a> or <a class="reference internal" href="read_data.html"><span class="doc">read_data</span></a> commands.</p>
|
|
<hr class="docutils" />
|
|
<p>The <em>molecule</em> style uses the molecule ID of each atom as its chunk
|
|
ID. <em>Nchunk</em> is set to the largest chunk ID. Note that this excludes
|
|
molecule IDs for atoms which are not in the specified group or
|
|
optional region.</p>
|
|
<p>There is no requirement that all atoms in a particular molecule are
|
|
assigned the same chunk ID (zero or non-zero), though you probably
|
|
want that to be the case, if you wish to compute a per-molecule
|
|
property. LAMMPS will issue a warning if that is not the case, but
|
|
only the first time that <em>Nchunk</em> is calculated.</p>
|
|
<p>Note that atoms with a molecule ID = 0, which may be non-molecular
|
|
solvent atoms, have an out-of-range chunk ID. These atoms are
|
|
discarded (not assigned to any chunk) or assigned to <em>Nchunk</em>,
|
|
depending on the value of the <em>discard</em> keyword.</p>
|
|
<hr class="docutils" />
|
|
<p>The <em>compute/fix/variable</em> styles set the chunk ID of each atom based
|
|
on a quantity calculated and stored by a compute, fix, or variable.
|
|
In each case, it must be a per-atom quantity. In each case the
|
|
referenced floating point values are converted to an integer chunk ID
|
|
as follows. The floating point value is truncated (rounded down) to
|
|
an integer value. If the integer value is <= 0, then a chunk ID of 0
|
|
is assigned to the atom. If the integer value is > 0, it becomes the
|
|
chunk ID to the atom. <em>Nchunk</em> is set to the largest chunk ID. Note
|
|
that this excludes atoms which are not in the specified group or
|
|
optional region.</p>
|
|
<p>If the style begins with “<a href="#id1"><span class="problematic" id="id2">c_</span></a>”, a compute ID must follow which has been
|
|
previously defined in the input script. If no bracketed integer is
|
|
appended, the per-atom vector calculated by the compute is used. If a
|
|
bracketed integer is appended, the Ith column of the per-atom array
|
|
calculated by the compute is used. Users can also write code for
|
|
their own compute styles and <a class="reference internal" href="Section_modify.html"><span class="doc">add them to LAMMPS</span></a>.</p>
|
|
<p>If the style begins with “<a href="#id3"><span class="problematic" id="id4">f_</span></a>”, a fix ID must follow which has been
|
|
previously defined in the input script. If no bracketed integer is
|
|
appended, the per-atom vector calculated by the fix is used. If a
|
|
bracketed integer is appended, the Ith column of the per-atom array
|
|
calculated by the fix is used. Note that some fixes only produce
|
|
their values on certain timesteps, which must be compatible with the
|
|
timestep on which this compute accesses the fix, else an error
|
|
results. Users can also write code for their own fix styles and <a class="reference internal" href="Section_modify.html"><span class="doc">add them to LAMMPS</span></a>.</p>
|
|
<p>If a value begins with “<a href="#id5"><span class="problematic" id="id6">v_</span></a>”, a variable name for an <em>atom</em> or
|
|
<em>atomfile</em> style <a class="reference internal" href="variable.html"><span class="doc">variable</span></a> must follow which has been
|
|
previously defined in the input script. Variables of style <em>atom</em> can
|
|
reference thermodynamic keywords and various per-atom attributes, or
|
|
invoke other computes, fixes, or variables when they are evaluated, so
|
|
this is a very general means of generating per-atom quantities to
|
|
treat as a chunk ID.</p>
|
|
<p>Normally, <em>Nchunk</em> = the number of chunks, is re-calculated every time
|
|
this fix is invoked, though the value may or may not change. As
|
|
explained below, the <em>nchunk</em> keyword can be set to <em>once</em> which means
|
|
<em>Nchunk</em> will never change.</p>
|
|
<p>If a <a class="reference internal" href="fix_ave_chunk.html"><span class="doc">fix ave/chunk</span></a> command uses this compute, it
|
|
can also turn off the re-calculation of <em>Nchunk</em> for one or more
|
|
windows of timesteps. The extent of the windows, during which Nchunk
|
|
is held constant, are determined by the <em>Nevery</em>, <em>Nrepeat</em>, <em>Nfreq</em>
|
|
values and the <em>ave</em> keyword setting that are used by the <a class="reference internal" href="fix_ave_chunk.html"><span class="doc">fix ave/chunk</span></a> command.</p>
|
|
<p>Specifically, if <em>ave</em> = <em>one</em>, then for each span of <em>Nfreq</em>
|
|
timesteps, <em>Nchunk</em> is held constant between the first timestep when
|
|
averaging is done (within the Nfreq-length window), and the last
|
|
timestep when averaging is done (multiple of Nfreq). If <em>ave</em> =
|
|
<em>running</em> or <em>window</em>, then <em>Nchunk</em> is held constant forever,
|
|
starting on the first timestep when the <a class="reference internal" href="fix_ave_chunk.html"><span class="doc">fix ave/chunk</span></a> command invokes this compute.</p>
|
|
<p>Note that multiple <a class="reference internal" href="fix_ave_chunk.html"><span class="doc">fix ave/chunk</span></a> commands can use
|
|
the same compute chunk/atom compute. However, the time windows they
|
|
induce for holding <em>Nchunk</em> constant must be identical, else an error
|
|
will be generated.</p>
|
|
<p>The various optional keywords operate as follows. Note that some of
|
|
them function differently or are ignored by different chunk styles.
|
|
Some of them also have different default values, depending on
|
|
the chunk style, as listed below.</p>
|
|
<p>The <em>region</em> keyword applies to all chunk styles. If used, an atom
|
|
must be in both the specified group and the specified geometric
|
|
<a class="reference internal" href="region.html"><span class="doc">region</span></a> to be assigned to a chunk.</p>
|
|
<hr class="docutils" />
|
|
<p>The <em>nchunk</em> keyword applies to all chunk styles. It specifies how
|
|
often <em>Nchunk</em> is recalculated, which in turn can affect the chunk IDs
|
|
assigned to individual atoms.</p>
|
|
<p>If <em>nchunk</em> is set to <em>once</em>, then <em>Nchunk</em> is only calculated once,
|
|
the first time this compute is invoked. If <em>nchunk</em> is set to
|
|
<em>every</em>, then <em>Nchunk</em> is re-calculated every time the compute is
|
|
invoked. Note that, as described above, the use of this compute
|
|
by the <a class="reference internal" href="fix_ave_chunk.html"><span class="doc">fix ave/chunk</span></a> command can override
|
|
the <em>every</em> setting.</p>
|
|
<p>The default values for <em>nchunk</em> are listed below and depend on the
|
|
chunk style and other system and keyword settings. They attempt to
|
|
represent typical use cases for the various chunk styles. The
|
|
<em>nchunk</em> value can always be set explicitly if desired.</p>
|
|
<hr class="docutils" />
|
|
<p>The <em>limit</em> keyword can be used to limit the calculated value of
|
|
<em>Nchunk</em> = the number of chunks. The limit is applied each time
|
|
<em>Nchunk</em> is calculated, which also limits the chunk IDs assigned to
|
|
any atom. The <em>limit</em> keyword is used by all chunk styles except the
|
|
<em>binning</em> styles, which ignore it. This is because the number of bins
|
|
can be tailored using the <em>bound</em> keyword (described below) which
|
|
effectively limits the size of <em>Nchunk</em>.</p>
|
|
<p>If <em>limit</em> is set to <em>Nc</em> = 0, then no limit is imposed on <em>Nchunk</em>,
|
|
though the <em>compress</em> keyword can still be used to reduce <em>Nchunk</em>, as
|
|
described below.</p>
|
|
<p>If <em>Nc</em> > 0, then the effect of the <em>limit</em> keyword depends on whether
|
|
the <em>compress</em> keyword is also used with a setting of <em>yes</em>, and
|
|
whether the <em>compress</em> keyword is specified before the <em>limit</em> keyword
|
|
or after.</p>
|
|
<p>In all cases, <em>Nchunk</em> is first calculated in the usual way for each
|
|
chunk style, as described above.</p>
|
|
<p>First, here is what occurs if <em>compress yes</em> is not set. If <em>limit</em>
|
|
is set to <em>Nc max</em>, then <em>Nchunk</em> is reset to the smaller of <em>Nchunk</em>
|
|
and <em>Nc</em>. If <em>limit</em> is set to <em>Nc exact</em>, then <em>Nchunk</em> is reset to
|
|
<em>Nc</em>, whether the original <em>Nchunk</em> was larger or smaller than <em>Nc</em>.
|
|
If <em>Nchunk</em> shrank due to the <em>limit</em> setting, then atom chunk IDs >
|
|
<em>Nchunk</em> will be reset to 0 or <em>Nchunk</em>, depending on the setting of
|
|
the <em>discard</em> keyword. If <em>Nchunk</em> grew, there will simply be some
|
|
chunks with no atoms assigned to them.</p>
|
|
<p>If <em>compress yes</em> is set, and the <em>compress</em> keyword comes before the
|
|
<em>limit</em> keyword, the compression operation is performed first, as
|
|
described below, which resets <em>Nchunk</em>. The <em>limit</em> keyword is then
|
|
applied to the new <em>Nchunk</em> value, exactly as described in the
|
|
preceeding paragraph. Note that in this case, all atoms will end up
|
|
with chunk IDs <= <em>Nc</em>, but their original values (e.g. molecule ID or
|
|
compute/fix/variable value) may have been > <em>Nc</em>, because of the
|
|
compression operation.</p>
|
|
<p>If <em>compress yes</em> is set, and the <em>compress</em> keyword comes after the
|
|
<em>limit</em> keyword, then the <em>limit</em> value of <em>Nc</em> is applied first to
|
|
the uncompressed value of <em>Nchunk</em>, but only if <em>Nc</em> < <em>Nchunk</em>
|
|
(whether <em>Nc max</em> or <em>Nc exact</em> is used). This effectively means all
|
|
atoms with chunk IDs > <em>Nc</em> have their chunk IDs reset to 0 or <em>Nc</em>,
|
|
depending on the setting of the <em>discard</em> keyword. The compression
|
|
operation is then performed, which may shrink <em>Nchunk</em> further. If
|
|
the new <em>Nchunk</em> < <em>Nc</em> and <em>limit</em> = <em>Nc exact</em> is specified, then
|
|
<em>Nchunk</em> is reset to <em>Nc</em>, which results in extra chunks with no atoms
|
|
assigned to them. Note that in this case, all atoms will end up with
|
|
chunk IDs <= <em>Nc</em>, and their original values (e.g. molecule ID or
|
|
compute/fix/variable value) will also have been <= <em>Nc</em>.</p>
|
|
<hr class="docutils" />
|
|
<p>The <em>ids</em> keyword applies to all chunk styles. If the setting is
|
|
<em>once</em> then the chunk IDs assigned to atoms the first time this
|
|
compute is invoked will be permanent, and never be re-computed.</p>
|
|
<p>If the setting is <em>nfreq</em> and if a <a class="reference internal" href="fix_ave_chunk.html"><span class="doc">fix ave/chunk</span></a>
|
|
command is using this compute, then in each of the <em>Nchunk</em> = constant
|
|
time windows (discussed above), the chunk ID’s assigned to atoms on
|
|
the first step of the time window will persist until the end of the
|
|
time window.</p>
|
|
<p>If the setting is <em>every</em>, which is the default, then chunk IDs are
|
|
re-calculated on any timestep this compute is invoked.</p>
|
|
<div class="admonition note">
|
|
<p class="first admonition-title">Note</p>
|
|
<p class="last">If you want the persistent chunk-IDs calculated by this compute
|
|
to be continuous when running from a <a class="reference internal" href="read_restart.html"><span class="doc">restart file</span></a>,
|
|
then you should use the same ID for this compute, as in the original
|
|
run. This is so that the fix this compute creates to store per-atom
|
|
quantities will also have the same ID, and thus be initialized
|
|
correctly with chunk IDs from the restart file.</p>
|
|
</div>
|
|
<hr class="docutils" />
|
|
<p>The <em>compress</em> keyword applies to all chunk styles and affects how
|
|
<em>Nchunk</em> is calculated, which in turn affects the chunk IDs assigned
|
|
to each atom. It is useful for converting a “sparse” set of chunk IDs
|
|
(with many IDs that have no atoms assigned to them), into a “dense”
|
|
set of IDs, where every chunk has one or more atoms assigned to it.</p>
|
|
<p>Two possible use cases are as follows. If a large simulation box is
|
|
mostly empty space, then the <em>binning</em> style may produce many bins
|
|
with no atoms. If <em>compress</em> is set to <em>yes</em>, only bins with atoms
|
|
will be contribute to <em>Nchunk</em>. Likewise, the <em>molecule</em> or
|
|
<em>compute/fix/variable</em> styles may produce large <em>Nchunk</em> values. For
|
|
example, the <a class="reference internal" href="compute_cluster_atom.html"><span class="doc">compute cluster/atom</span></a> command
|
|
assigns every atom an atom ID for one of the atoms it is clustered
|
|
with. For a million-atom system with 5 clusters, there would only be
|
|
5 unique chunk IDs, but the largest chunk ID might be 1 million,
|
|
resulting in <em>Nchunk</em> = 1 million. If <em>compress</em> is set to <em>yes</em>,
|
|
<em>Nchunk</em> will be reset to 5.</p>
|
|
<p>If <em>compress</em> is set to <em>no</em>, which is the default, no compression is
|
|
done. If it is set to <em>yes</em>, all chunk IDs with no atoms are removed
|
|
from the list of chunk IDs, and the list is sorted. The remaining
|
|
chunk IDs are renumbered from 1 to <em>Nchunk</em> where <em>Nchunk</em> is the new
|
|
length of the list. The chunk IDs assigned to each atom reflect
|
|
the new renumbering from 1 to <em>Nchunk</em>.</p>
|
|
<p>The original chunk IDs (before renumbering) can be accessed by the
|
|
<a class="reference internal" href="compute_property_chunk.html"><span class="doc">compute property/chunk</span></a> command and its
|
|
<em>id</em> keyword, or by the <a class="reference internal" href="fix_ave_chunk.html"><span class="doc">fix ave/chunk</span></a> command
|
|
which outputs the original IDs as one of the columns in its global
|
|
output array. For example, using the “compute cluster/atom” command
|
|
discussed above, the original 5 unique chunk IDs might be atom IDs
|
|
(27,4982,58374,857838,1000000). After compresion, these will be
|
|
renumbered to (1,2,3,4,5). The original values (27,...,1000000) can
|
|
be output to a file by the <a class="reference internal" href="fix_ave_chunk.html"><span class="doc">fix ave/chunk</span></a> command,
|
|
or by using the <a class="reference internal" href="fix_ave_time.html"><span class="doc">fix ave/time</span></a> command in
|
|
conjunction with the <a class="reference internal" href="compute_property_chunk.html"><span class="doc">compute property/chunk</span></a> command.</p>
|
|
<div class="admonition note">
|
|
<p class="first admonition-title">Note</p>
|
|
<p class="last">The compression operation requires global communication across
|
|
all processors to share their chunk ID values. It can require large
|
|
memory on every processor to store them, even after they are
|
|
compressed, if there are are a large number of unique chunk IDs with
|
|
atoms assigned to them. It uses a STL map to find unique chunk IDs
|
|
and store them in sorted order. Each time an atom is assigned a
|
|
compressed chunk ID, it must access the STL map. All of this means
|
|
that compression can be expensive, both in memory and CPU time. The
|
|
use of the <em>limit</em> keyword in conjunction with the <em>compress</em> keyword
|
|
can affect these costs, depending on which keyword is used first. So
|
|
use this option with care.</p>
|
|
</div>
|
|
<hr class="docutils" />
|
|
<p>The <em>discard</em> keyword applies to all chunk styles. It affects what
|
|
chunk IDs are assigned to atoms that do not match one of the valid
|
|
chunk IDs from 1 to <em>Nchunk</em>. Note that it does not apply to atoms
|
|
that are not in the specified group or optionally specified region.
|
|
Those atoms are always assigned a chunk ID = 0.</p>
|
|
<p>If the calculated chunk ID for an atom is not within the range 1 to
|
|
<em>Nchunk</em> then it is a “discard” atom. Note that <em>Nchunk</em> may have
|
|
been shrunk by the <em>limit</em> keyword. Or the <em>compress</em> keyword may
|
|
have eliminated chunk IDs that were valid before the compression took
|
|
place, and are now not in the compressed list. Also note that for the
|
|
<em>molecule</em> chunk style, if new molecules are added to the system,
|
|
their chunk IDs may exceed a previously calculated <em>Nchunk</em>.
|
|
Likewise, evaluation of a compute/fix/variable on a later timestep may
|
|
return chunk IDs that are invalid for the previously calculated
|
|
<em>Nchunk</em>.</p>
|
|
<p>All the chunk styles except the <em>binning</em> styles, must use <em>discard</em>
|
|
set to either <em>yes</em> or <em>no</em>. If <em>discard</em> is set to <em>yes</em>, which is
|
|
the default, then every “discard” atom has its chunk ID set to 0. If
|
|
<em>discard</em> is set to <em>no</em>, every “discard” atom has its chunk ID set to
|
|
<em>Nchunk</em>. I.e. it becomes part of the last chunk.</p>
|
|
<p>The <em>binning</em> styles use the <em>discard</em> keyword to decide whether to
|
|
discard atoms outside the spatial domain covered by bins, or to assign
|
|
them to the bin they are nearest to.</p>
|
|
<p>For the <em>bin/1d</em>, <em>bin/2d</em>, <em>bin/3d</em> styles the details are as
|
|
follows. If <em>discard</em> is set to <em>yes</em>, an out-of-domain atom will
|
|
have its chunk ID set to 0. If <em>discard</em> is set to <em>no</em>, the atom
|
|
will have its chunk ID set to the first or last bin in that dimension.
|
|
If <em>discard</em> is set to <em>mixed</em>, which is the default, it will only
|
|
have its chunk ID set to the first or last bin if bins extend to the
|
|
simulation box boundary in that dimension. This is the case if the
|
|
<em>bound</em> keyword settings are <em>lower</em> and <em>upper</em>, which is the
|
|
default. If the <em>bound</em> keyword settings are numeric values, then the
|
|
atom will have its chunk ID set to 0 if it is outside the bounds of
|
|
any bin. Note that in this case, it is possible that the first or
|
|
last bin extends beyond the numeric <em>bounds</em> settings, depending on
|
|
the specified <em>origin</em>. If this is the case, the chunk ID of the atom
|
|
is only set to 0 if it is outside the first or last bin, not if it is
|
|
simply outside the numeric <em>bounds</em> setting.</p>
|
|
<p>For the <em>bin/sphere</em> style the details are as follows. If <em>discard</em>
|
|
is set to <em>yes</em>, an out-of-domain atom will have its chunk ID set to
|
|
0. If <em>discard</em> is set to <em>no</em> or <em>mixed</em>, the atom will have its
|
|
chunk ID set to the first or last bin, i.e. the innermost or outermost
|
|
spherical shell. If the distance of the atom from the origin is less
|
|
than <em>rmin</em>, it will be assigned to the first bin. If the distance of
|
|
the atom from the origin is greater than <em>rmax</em>, it will be assigned
|
|
to the last bin.</p>
|
|
<p>For the <em>bin/cylinder</em> style the details are as follows. If <em>discard</em>
|
|
is set to <em>yes</em>, an out-of-domain atom will have its chunk ID set to
|
|
0. If <em>discard</em> is set to <em>no</em>, the atom will have its chunk ID set
|
|
to the first or last bin in both the radial and axis dimensions. If
|
|
<em>discard</em> is set to <em>mixed</em>, which is the default, the the radial
|
|
dimension is treated the same as for <em>discard</em> = no. But for the axis
|
|
dimensinon, it will only have its chunk ID set to the first or last
|
|
bin if bins extend to the simulation box boundary in the axis
|
|
dimension. This is the case if the <em>bound</em> keyword settings are
|
|
<em>lower</em> and <em>upper</em>, which is the default. If the <em>bound</em> keyword
|
|
settings are numeric values, then the atom will have its chunk ID set
|
|
to 0 if it is outside the bounds of any bin. Note that in this case,
|
|
it is possible that the first or last bin extends beyond the numeric
|
|
<em>bounds</em> settings, depending on the specified <em>origin</em>. If this is
|
|
the case, the chunk ID of the atom is only set to 0 if it is outside
|
|
the first or last bin, not if it is simply outside the numeric
|
|
<em>bounds</em> setting.</p>
|
|
<p>If <em>discard</em> is set to <em>no</em> or <em>mixed</em>, the atom will have its
|
|
chunk ID set to the first or last bin, i.e. the innermost or outermost
|
|
spherical shell. If the distance of the atom from the origin is less
|
|
than <em>rmin</em>, it will be assigned to the first bin. If the distance of
|
|
the atom from the origin is greater than <em>rmax</em>, it will be assigned
|
|
to the last bin.</p>
|
|
<hr class="docutils" />
|
|
<p>The <em>bound</em> keyword only applies to the <em>bin/1d</em>, <em>bin/2d</em>, <em>bin/3d</em>
|
|
styles and to the axis dimension of the <em>bin/cylinder</em> style;
|
|
otherwise it is ignored. It can be used one or more times to limit
|
|
the extent of bin coverage in a specified dimension, i.e. to only bin
|
|
a portion of the box. If the <em>lo</em> setting is <em>lower</em> or the <em>hi</em>
|
|
setting is <em>upper</em>, the bin extent in that direction extends to the
|
|
box boundary. If a numeric value is used for <em>lo</em> and/or <em>hi</em>, then
|
|
the bin extent in the <em>lo</em> or <em>hi</em> direction extends only to that
|
|
value, which is assumed to be inside (or at least near) the simulation
|
|
box boundaries, though LAMMPS does not check for this. Note that
|
|
using the <em>bound</em> keyword typically reduces the total number of bins
|
|
and thus the number of chunks <em>Nchunk</em>.</p>
|
|
<p>The <em>pbc</em> keyword only applies to the <em>bin/sphere</em> and <em>bin/cylinder</em>
|
|
styles. If set to <em>yes</em>, the distance an atom is from the sphere
|
|
origin or cylinder axis is calculated in a minimum image sense with
|
|
respect to periodic dimensions, when determining which bin the atom is
|
|
in. I.e. if x is a periodic dimension and the distance between the
|
|
atom and the sphere center in the x dimension is greater than 0.5 *
|
|
simulation box length in x, then a box length is subtracted to give a
|
|
distance < 0.5 * simulation box length. This allosws the sphere or
|
|
cylinder center to be near a box edge, and atoms on the other side of
|
|
the periodic box will still be close to the center point/axis. Note
|
|
that with a setting of <em>yes</em>, the outer sphere or cylinder radius must
|
|
also be <= 0.5 * simulation box length in any periodic dimension
|
|
except for the cylinder axis dimension, or an error is generated.</p>
|
|
<p>The <em>units</em> keyword only applies to the <em>binning</em> styles; otherwise it
|
|
is ignored. For the <em>bin/1d</em>, <em>bin/2d</em>, <em>bin/3d</em> styles, it
|
|
determines the meaning of the distance units used for the bin sizes
|
|
<em>delta</em> and for <em>origin</em> and <em>bounds</em> values if they are coordinate
|
|
values. For the <em>bin/sphere</em> style it determines the meaning of the
|
|
distance units used for <em>xorig</em>,<em>yorig</em>,<em>zorig</em> and the radii <em>srmin</em>
|
|
and <em>srmax</em>. For the <em>bin/cylinder</em> style it determines the meaning
|
|
of the distance units used for <em>delta</em>,<em>c1</em>,<em>c2</em> and the radii <em>crmin</em>
|
|
and <em>crmax</em>.</p>
|
|
<p>For orthogonal simulation boxes, any of the 3 options may
|
|
be used. For non-orthogonal (triclinic) simulation boxes, only the
|
|
<em>reduced</em> option may be used.</p>
|
|
<p>A <em>box</em> value selects standard distance units as defined by the
|
|
<a class="reference internal" href="units.html"><span class="doc">units</span></a> command, e.g. Angstroms for units = real or metal.
|
|
A <em>lattice</em> value means the distance units are in lattice spacings.
|
|
The <a class="reference internal" href="lattice.html"><span class="doc">lattice</span></a> command must have been previously used to
|
|
define the lattice spacing. A <em>reduced</em> value means normalized
|
|
unitless values between 0 and 1, which represent the lower and upper
|
|
faces of the simulation box respectively. Thus an <em>origin</em> value of
|
|
0.5 means the center of the box in any dimension. A <em>delta</em> value of
|
|
0.1 means 10 bins span the box in that dimension.</p>
|
|
<p>Note that for the <em>bin/sphere</em> style, the radii <em>srmin</em> and <em>srmax</em> are
|
|
scaled by the lattice spacing or reduced value of the <em>x</em> dimension.</p>
|
|
<p>Note that for the <em>bin/cylinder</em> style, the radii <em>crmin</em> and <em>crmax</em>
|
|
are scaled by the lattice spacing or reduced value of the 1st
|
|
dimension perpendicular to the cylinder axis. E.g. y for an x-axis
|
|
cylinder, x for a y-axis cylinder, and x for a z-axis cylinder.</p>
|
|
<hr class="docutils" />
|
|
<p><strong>Output info:</strong></p>
|
|
<p>This compute calculates a per-atom vector, which can be accessed by
|
|
any command that uses per-atom values from a compute as input. See
|
|
<a class="reference internal" href="Section_howto.html#howto-15"><span class="std std-ref">Section_howto 15</span></a> for an overview of
|
|
LAMMPS output options.</p>
|
|
<p>The per-atom vector values are unitless chunk IDs, ranging from 1 to
|
|
<em>Nchunk</em> (inclusive) for atoms assigned to chunks, and 0 for atoms not
|
|
belonging to a chunk.</p>
|
|
</div>
|
|
<div class="section" id="restrictions">
|
|
<h2>Restrictions</h2>
|
|
<p>Even if the <em>nchunk</em> keyword is set to <em>once</em>, the chunk IDs assigned
|
|
to each atom are not stored in a restart files. This means you cannot
|
|
expect those assignments to persist in a restarted simulation.
|
|
Instead you must re-specify this command and assign atoms to chunks when
|
|
the restarted simulation begins.</p>
|
|
</div>
|
|
<div class="section" id="related-commands">
|
|
<h2>Related commands</h2>
|
|
<p><a class="reference internal" href="fix_ave_chunk.html"><span class="doc">fix ave/chunk</span></a></p>
|
|
</div>
|
|
<div class="section" id="default">
|
|
<h2>Default</h2>
|
|
<p>The option defaults are as follows:</p>
|
|
<ul class="simple">
|
|
<li>region = none</li>
|
|
<li>nchunk = every, if compress is yes, overriding other defaults listed here</li>
|
|
<li>nchunk = once, for type style</li>
|
|
<li>nchunk = once, for mol style if region is none</li>
|
|
<li>nchunk = every, for mol style if region is set</li>
|
|
<li>nchunk = once, for binning style if the simulation box size is static or units = reduced</li>
|
|
<li>nchunk = every, for binning style if the simulation box size is dynamic and units is lattice or box</li>
|
|
<li>nchunk = every, for compute/fix/variable style</li>
|
|
<li>limit = 0</li>
|
|
<li>ids = every</li>
|
|
<li>compress = no</li>
|
|
<li>discard = yes, for all styles except binning</li>
|
|
<li>discard = mixed, for binning styles</li>
|
|
<li>bound = lower and upper in all dimensions</li>
|
|
<li>pbc = no</li>
|
|
<li>units = lattice</li>
|
|
</ul>
|
|
</div>
|
|
</div>
|
|
|
|
|
|
</div>
|
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</div>
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