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<div class="lammps_version">Version: <b>19 Nov 2024</b></div>
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<p class="caption" role="heading"><span class="caption-text">User Guide</span></p>
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<li class="toctree-l1"><a class="reference internal" href="Intro.html">1. Introduction</a></li>
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<li class="toctree-l1"><a class="reference internal" href="Install.html">2. Install LAMMPS</a></li>
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<li class="toctree-l1"><a class="reference internal" href="Build.html">3. Build LAMMPS</a></li>
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<li class="toctree-l1"><a class="reference internal" href="Run_head.html">4. Run LAMMPS</a></li>
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<li class="toctree-l1"><a class="reference internal" href="Commands.html">5. Commands</a></li>
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<li class="toctree-l1"><a class="reference internal" href="Packages.html">6. Optional packages</a></li>
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<li class="toctree-l1"><a class="reference internal" href="Speed.html">7. Accelerate performance</a></li>
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<li class="toctree-l1"><a class="reference internal" href="Howto.html">8. Howto discussions</a></li>
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<li class="toctree-l1"><a class="reference internal" href="Examples.html">9. Example scripts</a></li>
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<li class="toctree-l1"><a class="reference internal" href="Tools.html">10. Auxiliary tools</a></li>
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<li class="toctree-l1"><a class="reference internal" href="Errors.html">11. Errors</a></li>
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<p class="caption" role="heading"><span class="caption-text">Programmer Guide</span></p>
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<li class="toctree-l1"><a class="reference internal" href="Library.html">1. LAMMPS Library Interfaces</a></li>
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<li class="toctree-l1 current"><a class="reference internal" href="Modify.html">3. Modifying & extending LAMMPS</a><ul class="current">
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<li class="toctree-l2 current"><a class="current reference internal" href="#">3.1. Overview</a></li>
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<li class="toctree-l2"><a class="reference internal" href="Modify_contribute.html">3.2. Submitting new features for inclusion in LAMMPS</a></li>
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<li class="toctree-l2"><a class="reference internal" href="Modify_requirements.html">3.3. Requirements for contributions to LAMMPS</a></li>
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<li class="toctree-l2"><a class="reference internal" href="Modify_style.html">3.4. LAMMPS programming style</a></li>
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<li class="toctree-l2"><a class="reference internal" href="Modify_atom.html">3.5. Atom styles</a></li>
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<li class="toctree-l2"><a class="reference internal" href="Modify_pair.html">3.6. Pair styles</a></li>
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<li class="toctree-l2"><a class="reference internal" href="Modify_bond.html">3.7. Bond, angle, dihedral, improper styles</a></li>
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<li class="toctree-l2"><a class="reference internal" href="Modify_compute.html">3.8. Compute styles</a></li>
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<li class="toctree-l2"><a class="reference internal" href="Modify_fix.html">3.9. Fix styles</a></li>
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<li class="toctree-l2"><a class="reference internal" href="Modify_command.html">3.10. Input script command style</a></li>
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<li class="toctree-l2"><a class="reference internal" href="Modify_dump.html">3.11. Dump styles</a></li>
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<li class="toctree-l2"><a class="reference internal" href="Modify_kspace.html">3.12. Kspace styles</a></li>
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<li class="toctree-l2"><a class="reference internal" href="Modify_min.html">3.13. Minimization styles</a></li>
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<li class="toctree-l2"><a class="reference internal" href="Modify_region.html">3.14. Region styles</a></li>
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<li class="toctree-l2"><a class="reference internal" href="Modify_body.html">3.15. Body styles</a></li>
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<li class="toctree-l2"><a class="reference internal" href="Modify_gran_sub_mod.html">3.16. Granular Sub-Model styles</a></li>
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<li class="toctree-l2"><a class="reference internal" href="Modify_thermo.html">3.17. Thermodynamic output options</a></li>
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<li class="toctree-l2"><a class="reference internal" href="Modify_variable.html">3.18. Variable options</a></li>
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<li class="toctree-l1"><a class="reference internal" href="Developer.html">4. Information for Developers</a></li>
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<li class="toctree-l1"><a class="reference internal" href="pairs.html">Pair Styles</a></li>
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<li class="toctree-l1"><a class="reference internal" href="dihedrals.html">Dihedral Styles</a></li>
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<li class="toctree-l1"><a class="reference internal" href="impropers.html">Improper Styles</a></li>
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<li class="toctree-l1"><a class="reference internal" href="dumps.html">Dump Styles</a></li>
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<li class="toctree-l1"><a class="reference internal" href="fix_modify_atc_commands.html">fix_modify AtC commands</a></li>
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<li class="toctree-l1"><a class="reference internal" href="Bibliography.html">Bibliography</a></li>
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<p><span class="math notranslate nohighlight">\(\renewcommand{\AA}{\text{Å}}\)</span></p>
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<section id="overview">
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<h1><span class="section-number">3.1. </span>Overview<a class="headerlink" href="#overview" title="Link to this heading"></a></h1>
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<p>The best way to add a new feature to LAMMPS is to find a similar
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feature and look at the corresponding source and header files to
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figure out what it does. You will need some knowledge of C++ to
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understand the high-level structure of LAMMPS and its class
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organization. Functions (class methods) that do actual computations
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are mostly written in C-style code and operate on simple C-style data
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structures (vectors and arrays). A high-level overview of the
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programming style choices in LAMMPS is <a class="reference internal" href="Developer_code_design.html"><span class="doc">given elsewhere</span></a>.</p>
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<p>Most of the new features described on the <a class="reference internal" href="Modify.html"><span class="doc">Modify</span></a> doc
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page require you to write a new C++ derived class (excluding exceptions
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described below, this can often be done by making small edits to existing
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files). Creating a new class requires 2 files, a source code file
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(*.cpp) and a header file (*.h). The derived class must provide
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certain methods to work as a new option. Depending on how different
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your new feature is compared to existing features, you can either
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derive from the base class itself, or from a derived class that
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already exists. Enabling LAMMPS to invoke the new class is as simple
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as putting the two source files in the src directory and re-building
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LAMMPS.</p>
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<p>The advantage of C++ and its object-orientation is that all the code
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and variables needed to define the new feature are in the 2 files you
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write. Thus, it should not make the rest of LAMMPS more complex or
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cause bugs through unwanted side effects.</p>
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<p>Here is a concrete example. Suppose you write 2 files
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<code class="docutils literal notranslate"><span class="pre">pair_foo.cpp</span></code> and <code class="docutils literal notranslate"><span class="pre">pair_foo.h</span></code> that define a new class
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<code class="docutils literal notranslate"><span class="pre">PairFoo</span></code> which computes pairwise potentials described in the
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classic 1997 <a class="reference internal" href="#foo"><span class="std std-ref">paper</span></a> by Foo, <em>et al.</em> If you wish to invoke
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those potentials in a LAMMPS input script with a command like:</p>
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<div class="highlight-LAMMPS notranslate"><div class="highlight"><pre><span></span><span class="k">pair_style</span><span class="w"> </span><span class="n">foo</span><span class="w"> </span><span class="m">0.1</span><span class="w"> </span><span class="m">3.5</span>
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</pre></div>
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</div>
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<p>then your <code class="docutils literal notranslate"><span class="pre">pair_foo.h</span></code> file should be structured as follows:</p>
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<div class="highlight-c++ notranslate"><div class="highlight"><pre><span></span><span class="cp">#ifdef PAIR_CLASS</span>
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<span class="c1">// clang-format off</span>
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<span class="n">PairStyle</span><span class="p">(</span><span class="n">foo</span><span class="p">,</span><span class="n">PairFoo</span><span class="p">);</span>
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<span class="cp">#else</span>
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<span class="c1">// clang-format on</span>
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<span class="p">...</span>
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<span class="p">(</span><span class="k">class</span><span class="w"> </span><span class="nc">definition</span><span class="w"> </span><span class="k">for</span><span class="w"> </span><span class="n">PairFoo</span><span class="p">)</span>
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<span class="p">...</span>
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<span class="cp">#endif</span>
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</pre></div>
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</div>
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<p>where “foo” is the style keyword in the pair_style command, and
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<code class="docutils literal notranslate"><span class="pre">PairFoo</span></code> is the class name defined in your <code class="docutils literal notranslate"><span class="pre">pair_foo.cpp</span></code> and
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<code class="docutils literal notranslate"><span class="pre">pair_foo.h</span></code> files.</p>
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<p>When you re-build LAMMPS, your new pairwise potential becomes part of
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the executable and can be invoked with a pair_style command like the
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example above. Arguments like 0.1 and 3.5 can be defined and
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processed by your new class.</p>
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<p>As illustrated by this example, many features referred to in the
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LAMMPS documentation are called a “style” of a particular command.</p>
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<p>The <a class="reference internal" href="Modify.html"><span class="doc">Modify page</span></a> lists all the common styles in LAMMPS,
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and discusses the header file for the base class that these styles
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derive from. Public variables in that file can be used and set by the
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derived classes, and may also be used by the base class. Sometimes
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they are also accessed by the rest of LAMMPS. Pure functions, which
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means functions declared as virtual in the base class header file and
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which are also set to 0, are functions you <strong>must</strong> implement in your
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new derived class to give it the functionality LAMMPS expects. Virtual
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functions that are not set to 0 are functions you may override or not.
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Those are usually defined with an empty function body.</p>
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<p>Additionally, new output options can be added directly to the
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thermo.cpp, dump_custom.cpp, and variable.cpp files. These are also
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listed on the <a class="reference internal" href="Modify.html"><span class="doc">Modify page</span></a>.</p>
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<p>Here are additional guidelines for modifying LAMMPS and adding new
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functionality:</p>
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<ul class="simple">
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<li><p>Think about whether what you want to do would be better as a pre- or
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post-processing step. Many computations are more easily and more
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quickly done that way.</p></li>
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<li><p>Do not try to do anything within the timestepping of a run that is not
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parallel. For example, do not accumulate a bunch of data on a single
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processor and analyze it. That would run the risk of seriously degrading
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the parallel efficiency.</p></li>
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<li><p>If your new feature reads arguments or writes output, make sure you
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follow the unit conventions discussed by the <a class="reference internal" href="units.html"><span class="doc">units</span></a>
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command.</p></li>
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</ul>
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<hr class="docutils" />
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<p id="foo"><strong>(Foo)</strong> Foo, Morefoo, and Maxfoo, J of Classic Potentials, 75, 345 (1997).</p>
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</section>
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