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<li class="toctree-l3"><a class="reference internal" href="Howto_chunk.html">8.3.2. Use chunks to calculate system properties</a></li>
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<li class="toctree-l3"><a class="reference internal" href="Howto_temperature.html">8.3.4. Calculate temperature</a></li>
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<li class="toctree-l3"><a class="reference internal" href="Howto_elastic.html">8.3.5. Calculate elastic constants</a></li>
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<li class="toctree-l3"><a class="reference internal" href="Howto_kappa.html">8.3.6. Calculate thermal conductivity</a></li>
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<li class="toctree-l3"><a class="reference internal" href="Howto_structured_data.html">8.3.9. Output structured data from LAMMPS</a></li>
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<li class="breadcrumb-item active"><span class="section-number">8.3.3. </span>Using distributed grids</li>
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<p><span class="math notranslate nohighlight">\(\renewcommand{\AA}{\text{Å}}\)</span></p>
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<section id="using-distributed-grids">
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<h1><span class="section-number">8.3.3. </span>Using distributed grids<a class="headerlink" href="#using-distributed-grids" title="Link to this heading"></a></h1>
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<div class="versionadded">
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<p><span class="versionmodified added">Added in version 22Dec2022.</span></p>
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</div>
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<p>LAMMPS has internal capabilities to create uniformly spaced grids
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which overlay the simulation domain. For 2d and 3d simulations these
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are 2d and 3d grids respectively. Conceptually a grid can be thought
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of as a collection of grid cells. Each grid cell can store one or
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more values (data).</p>
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<p>The grid cells and data they store are distributed across processors.
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Each processor owns the grid cells (and data) whose center points lie
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within the spatial subdomain of the processor. If needed for its
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computations, a processor may also store ghost grid cells with their
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data.</p>
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<p>Distributed grids can overlay orthogonal or triclinic simulation
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boxes; see the <a class="reference internal" href="Howto_triclinic.html"><span class="doc">Howto triclinic</span></a> doc page for
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an explanation of the latter. For a triclinic box, the grid cell
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shape conforms to the shape of the simulation domain,
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e.g. parallelograms instead of rectangles in 2d.</p>
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<p>If the box size or shape changes during a simulation, the grid changes
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with it, so that it always overlays the entire simulation domain. For
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non-periodic dimensions, the grid size in that dimension matches the
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box size, as set by the <a class="reference internal" href="boundary.html"><span class="doc">boundary</span></a> command for fixed
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or shrink-wrapped boundaries.</p>
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<p>If load-balancing is invoked by the <a class="reference internal" href="balance.html"><span class="doc">balance</span></a> or
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<a class="reference internal" href="fix_balance.html"><span class="doc">fix balance</span></a> commands, then the subdomain owned
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by a processor can change which may also change which grid cells they
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own.</p>
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<p>Post-processing and visualization of grid cell data can be enabled by
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the <a class="reference internal" href="dump.html"><span class="doc">dump grid</span></a>, <a class="reference internal" href="dump.html"><span class="doc">dump grid/vtk</span></a>, and
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<a class="reference internal" href="dump_image.html"><span class="doc">dump image</span></a> commands. The latter has an optional
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<em>grid</em> keyword. The <a class="reference external" href="https://www.ovito.org">OVITO visualization tool</a> also plans (as of Nov 2022) to add support
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for visualizing grid cell data (along with atoms) using <a class="reference internal" href="dump.html"><span class="doc">dump
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grid</span></a> output files as input.</p>
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<div class="admonition note">
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<p class="admonition-title">Note</p>
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<p>For developers, distributed grids are implemented within the code via
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two classes: Grid2d and Grid3d. These partition the grid across
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processors and have methods which allow forward and reverse
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communication of ghost grid data as well as load balancing. If you
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write a new compute or fix which needs a distributed grid, these are
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the classes to look at. A new pair style could use a distributed
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grid by having a fix define it. Please see the section on
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<a class="reference internal" href="Developer_grid.html"><span class="doc">using distributed grids within style classes</span></a>
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for a detailed description.</p>
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</div>
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<hr class="docutils" />
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<p>These are the commands which currently define or use distributed
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grids:</p>
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<ul class="simple">
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<li><p><a class="reference internal" href="fix_ttm.html"><span class="doc">fix ttm/grid</span></a> - store electron temperature on grid</p></li>
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<li><p><a class="reference internal" href="fix_ave_grid.html"><span class="doc">fix ave/grid</span></a> - time average per-atom or per-grid values</p></li>
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<li><p><a class="reference internal" href="compute_property_grid.html"><span class="doc">compute property/grid</span></a> - generate grid IDs and coords</p></li>
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<li><p><a class="reference internal" href="dump.html"><span class="doc">dump grid</span></a> - output per-grid values in LAMMPS format</p></li>
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<li><p><a class="reference internal" href="dump.html"><span class="doc">dump grid/vtk</span></a> - output per-grid values in VTK format</p></li>
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<li><p><a class="reference internal" href="dump_image.html"><span class="doc">dump image grid</span></a> - include colored grid in output images</p></li>
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<li><p><a class="reference internal" href="pair_amoeba.html"><span class="doc">pair_style amoeba</span></a> - FFT grids</p></li>
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<li><p><a class="reference internal" href="kspace_style.html"><span class="doc">kspace_style pppm</span></a> (and variants) - FFT grids</p></li>
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<li><p><a class="reference internal" href="kspace_style.html"><span class="doc">kspace_style msm</span></a> (and variants) - MSM grids</p></li>
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</ul>
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<p>The grids used by the <a class="reference internal" href="kspace_style.html"><span class="doc">kspace_style</span></a> can not be
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referenced by an input script. However the grids and data created and
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used by the other commands can be.</p>
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<p>A compute or fix command may create one or more grids (of different
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sizes). Each grid can store one or more data fields. A data field
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can be a single value per grid point (per-grid vector) or multiple
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values per grid point (per-grid array). See the <a class="reference internal" href="Howto_output.html"><span class="doc">Howto output</span></a> doc page for an explanation of how per-grid data can
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be generated by some commands and used by other commands.</p>
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<p>A command accesses grid data from a compute or fix using a <em>grid
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reference</em> with the following syntax:</p>
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<ul class="simple">
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<li><p>c_ID:gname:dname</p></li>
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<li><p>c_ID:gname:dname[I]</p></li>
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<li><p>f_ID:gname:dname</p></li>
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<li><p>f_ID:gname:dname[I]</p></li>
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</ul>
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<p>The prefix “c_” or “f_” refers to the ID of the compute or fix; gname is
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the name of the grid, which is assigned by the compute or fix; dname is
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the name of the data field, which is also assigned by the compute or
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fix.</p>
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<p>If the data field is a per-grid vector (one value per grid point),
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then no brackets are used to access the values. If the data field is
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a per-grid array (multiple values per grid point), then brackets are
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used to specify the column I of the array. I ranges from 1 to Ncol
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inclusive, where Ncol is the number of columns in the array and is
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defined by the compute or fix.</p>
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<p>Currently, there are no per-grid variables implemented in LAMMPS. We
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may add this feature at some point.</p>
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</section>
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