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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_accelerate.html">5. Accelerating LAMMPS performance</a></li>
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<li>pair_coeff command</li>
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<div class="section" id="pair-coeff-command">
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<span id="index-0"></span><h1>pair_coeff command<a class="headerlink" href="#pair-coeff-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>pair_coeff I J args
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</pre></div>
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</div>
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<ul class="simple">
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<li>I,J = atom types (see asterisk form below)</li>
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<li>args = coefficients for one or more pairs of atom types</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>pair_coeff 1 2 1.0 1.0 2.5
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pair_coeff 2 * 1.0 1.0
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pair_coeff 3* 1*2 1.0 1.0 2.5
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pair_coeff * * 1.0 1.0
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pair_coeff * * nialhjea 1 1 2
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pair_coeff * 3 morse.table ENTRY1
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pair_coeff 1 2 lj/cut 1.0 1.0 2.5 (for pair_style hybrid)
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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>Specify the pairwise force field coefficients for one or more pairs of
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atom types. The number and meaning of the coefficients depends on the
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pair style. Pair coefficients can also be set in the data file read
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by the <a class="reference internal" href="read_data.html"><em>read_data</em></a> command or in a restart file.</p>
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<p>I and J can be specified in one of two ways. Explicit numeric values
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can be used for each, as in the 1st example above. I <= J is
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required. LAMMPS sets the coefficients for the symmetric J,I
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interaction to the same values.</p>
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<p>A wildcard asterisk can be used in place of or in conjunction with the
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I,J arguments to set the coefficients for multiple pairs of atom
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types. This takes the form “*” or “<em>n” or “n</em>” or “m*n”. If N = the
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number of atom types, then an asterisk with no numeric values means all
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types from 1 to N. A leading asterisk means all types from 1 to n
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(inclusive). A trailing asterisk means all types from n to N
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(inclusive). A middle asterisk means all types from m to n
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(inclusive). Note that only type pairs with I <= J are considered; if
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asterisks imply type pairs where J < I, they are ignored.</p>
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<p>Note that a pair_coeff command can override a previous setting for the
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same I,J pair. For example, these commands set the coeffs for all I,J
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pairs, then overwrite the coeffs for just the I,J = 2,3 pair:</p>
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<div class="highlight-python"><div class="highlight"><pre>pair_coeff * * 1.0 1.0 2.5
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pair_coeff 2 3 2.0 1.0 1.12
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</pre></div>
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</div>
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<p>A line in a data file that specifies pair coefficients uses the exact
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same format as the arguments of the pair_coeff command in an input
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script, with the exception of the I,J type arguments. In each line of
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the “Pair Coeffs” section of a data file, only a single type I is
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specified, which sets the coefficients for type I interacting with
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type I. This is because the section has exactly N lines, where N =
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the number of atom types. For this reason, the wild-card asterisk
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should also not be used as part of the I argument. Thus in a data
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file, the line corresponding to the 1st example above would be listed
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as</p>
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<div class="highlight-python"><div class="highlight"><pre>2 1.0 1.0 2.5
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</pre></div>
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</div>
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<p>For many potentials, if coefficients for type pairs with I != J are
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not set explicitly by a pair_coeff command, the values are inferred
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from the I,I and J,J settings by mixing rules; see the
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<a class="reference internal" href="pair_modify.html"><em>pair_modify</em></a> command for a discussion. Details on
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this option as it pertains to individual potentials are described on
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the doc page for the potential.</p>
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<p>Many pair styles, typically for many-body potentials, use tabulated
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potential files as input, when specifying the pair_coeff command.
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Potential files provided with LAMMPS are in the potentials directory
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of the distribution. For some potentials, such as EAM, other archives
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of suitable files can be found on the Web. They can be used with
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LAMMPS so long as they are in the format LAMMPS expects, as discussed
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on the individual doc pages.</p>
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<p>When a pair_coeff command using a potential file is specified, LAMMPS
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looks for the potential file in 2 places. First it looks in the
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location specified. E.g. if the file is specified as “niu3.eam”, it
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is looked for in the current working directory. If it is specified as
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”../potentials/niu3.eam”, then it is looked for in the potentials
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directory, assuming it is a sister directory of the current working
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directory. If the file is not found, it is then looked for in the
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directory specified by the LAMMPS_POTENTIALS environment variable.
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Thus if this is set to the potentials directory in the LAMMPS distro,
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then you can use those files from anywhere on your system, without
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copying them into your working directory. Environment variables are
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set in different ways for different shells. Here are example settings
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for</p>
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<div class="highlight-python"><div class="highlight"><pre>csh, tcsh:
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% setenv LAMMPS_POTENTIALS /path/to/lammps/potentials
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</pre></div>
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</div>
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<div class="highlight-python"><div class="highlight"><pre>bash:
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% export LAMMPS_POTENTIALS=/path/to/lammps/potentials
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</pre></div>
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</div>
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<pre class="literal-block">
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Windows:
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% set LAMMPS_POTENTIALS="C:Path to LAMMPSPotentials
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</pre>
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<hr class="docutils" />
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<p>The alphabetic list of pair styles defined in LAMMPS is given on the
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<a class="reference internal" href="pair_style.html"><em>pair_style</em></a> doc page. They are also given in more
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compact form in the pair section of <a class="reference internal" href="Section_commands.html#cmd-5"><span>this page</span></a>.</p>
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<p>Click on the style to display the formula it computes, arguments
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specified in the pair_style command, and coefficients specified by the
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associated <a class="reference internal" href=""><em>pair_coeff</em></a> command.</p>
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<p>Note that there are also additional pair styles (not listed on the
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<a class="reference internal" href="pair_style.html"><em>pair_style</em></a> doc page) submitted by users which are
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included in the LAMMPS distribution. The list of these with links to
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the individual styles are given in the pair section of <a class="reference internal" href="Section_commands.html#cmd-5"><span>this page</span></a>.</p>
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<p>There are also additional accelerated pair styles (not listed on the
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<a class="reference internal" href="pair_style.html"><em>pair_style</em></a> doc page) included in the LAMMPS
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distribution for faster performance on CPUs and GPUs. The list of
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these with links to the individual styles are given in the pair
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section of <a class="reference internal" href="Section_commands.html#cmd-5"><span>this page</span></a>.</p>
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</div>
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<hr class="docutils" />
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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 command must come after the simulation box is defined by a
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<a class="reference internal" href="read_data.html"><em>read_data</em></a>, <a class="reference internal" href="read_restart.html"><em>read_restart</em></a>, or
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<a class="reference internal" href="create_box.html"><em>create_box</em></a> command.</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="pair_style.html"><em>pair_style</em></a>, <a class="reference internal" href="pair_modify.html"><em>pair_modify</em></a>,
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<a class="reference internal" href="read_data.html"><em>read_data</em></a>, <a class="reference internal" href="read_restart.html"><em>read_restart</em></a>,
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<a class="reference internal" href="pair_write.html"><em>pair_write</em></a></p>
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<p><strong>Default:</strong> none</p>
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