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<li class="toctree-l1"><a class="reference internal" href="Section_intro.html">1. Introduction</a></li>
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<div class="section" id="pair-style-sw-command">
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<span id="index-0"></span><h1>pair_style sw command<a class="headerlink" href="#pair-style-sw-command" title="Permalink to this headline">¶</a></h1>
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</div>
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<div class="section" id="pair-style-sw-cuda-command">
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<h1>pair_style sw/cuda command<a class="headerlink" href="#pair-style-sw-cuda-command" title="Permalink to this headline">¶</a></h1>
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</div>
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<div class="section" id="pair-style-sw-gpu-command">
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<h1>pair_style sw/gpu command<a class="headerlink" href="#pair-style-sw-gpu-command" title="Permalink to this headline">¶</a></h1>
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</div>
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<div class="section" id="pair-style-sw-intel-command">
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<h1>pair_style sw/intel command<a class="headerlink" href="#pair-style-sw-intel-command" title="Permalink to this headline">¶</a></h1>
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</div>
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<div class="section" id="pair-style-sw-kk-command">
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<h1>pair_style sw/kk command<a class="headerlink" href="#pair-style-sw-kk-command" title="Permalink to this headline">¶</a></h1>
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</div>
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<div class="section" id="pair-style-sw-omp-command">
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<h1>pair_style sw/omp command<a class="headerlink" href="#pair-style-sw-omp-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_style sw
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</pre></div>
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</div>
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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_style sw
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pair_coeff * * si.sw Si
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pair_coeff * * GaN.sw Ga N Ga
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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>The <em>sw</em> style computes a 3-body <a class="reference internal" href="#stillinger"><span>Stillinger-Weber</span></a>
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potential for the energy E of a system of atoms as</p>
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<img alt="_images/pair_sw.jpg" class="align-center" src="_images/pair_sw.jpg" />
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<p>where phi2 is a two-body term and phi3 is a three-body term. The
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summations in the formula are over all neighbors J and K of atom I
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within a cutoff distance = a*sigma.</p>
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<p>Only a single pair_coeff command is used with the <em>sw</em> style which
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specifies a Stillinger-Weber potential file with parameters for all
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needed elements. These are mapped to LAMMPS atom types by specifying
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N additional arguments after the filename in the pair_coeff command,
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where N is the number of LAMMPS atom types:</p>
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<ul class="simple">
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<li>filename</li>
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<li>N element names = mapping of SW elements to atom types</li>
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</ul>
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<p>See the <a class="reference internal" href="pair_coeff.html"><em>pair_coeff</em></a> doc page for alternate ways
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to specify the path for the potential file.</p>
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<p>As an example, imagine a file SiC.sw has Stillinger-Weber values for
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Si and C. If your LAMMPS simulation has 4 atoms types and you want
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the 1st 3 to be Si, and the 4th to be C, you would use the following
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pair_coeff command:</p>
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<div class="highlight-python"><div class="highlight"><pre>pair_coeff * * SiC.sw Si Si Si C
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</pre></div>
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</div>
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<p>The 1st 2 arguments must be * * so as to span all LAMMPS atom types.
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The first three Si arguments map LAMMPS atom types 1,2,3 to the Si
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element in the SW file. The final C argument maps LAMMPS atom type 4
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to the C element in the SW file. If a mapping value is specified as
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NULL, the mapping is not performed. This can be used when a <em>sw</em>
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potential is used as part of the <em>hybrid</em> pair style. The NULL values
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are placeholders for atom types that will be used with other
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potentials.</p>
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<p>Stillinger-Weber files in the <em>potentials</em> directory of the LAMMPS
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distribution have a ”.sw” suffix. Lines that are not blank or
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comments (starting with #) define parameters for a triplet of
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elements. The parameters in a single entry correspond to the two-body
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and three-body coefficients in the formula above:</p>
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<ul class="simple">
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<li>element 1 (the center atom in a 3-body interaction)</li>
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<li>element 2</li>
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<li>element 3</li>
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<li>epsilon (energy units)</li>
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<li>sigma (distance units)</li>
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<li>a</li>
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<li>lambda</li>
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<li>gamma</li>
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<li>costheta0</li>
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<li>A</li>
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<li>B</li>
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<li>p</li>
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<li>q</li>
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<li>tol</li>
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</ul>
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<p>The A, B, p, and q parameters are used only for two-body
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interactions. The lambda and costheta0 parameters are used only for
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three-body interactions. The epsilon, sigma and a parameters are used
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for both two-body and three-body interactions. gamma is used only in the
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three-body interactions, but is defined for pairs of atoms.
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The non-annotated parameters are unitless.</p>
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<p>LAMMPS introduces an additional performance-optimization parameter tol
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that is used for both two-body and three-body interactions. In the
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Stillinger-Weber potential, the interaction energies become negligibly
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small at atomic separations substantially less than the theoretical
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cutoff distances. LAMMPS therefore defines a virtual cutoff distance
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based on a user defined tolerance tol. The use of the virtual cutoff
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distance in constructing atom neighbor lists can significantly reduce
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the neighbor list sizes and therefore the computational cost. LAMMPS
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provides a <em>tol</em> value for each of the three-body entries so that they
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can be separately controlled. If tol = 0.0, then the standard
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Stillinger-Weber cutoff is used.</p>
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<p>The Stillinger-Weber potential file must contain entries for all the
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elements listed in the pair_coeff command. It can also contain
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entries for additional elements not being used in a particular
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simulation; LAMMPS ignores those entries.</p>
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<p>For a single-element simulation, only a single entry is required
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(e.g. SiSiSi). For a two-element simulation, the file must contain 8
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entries (for SiSiSi, SiSiC, SiCSi, SiCC, CSiSi, CSiC, CCSi, CCC), that
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specify SW parameters for all permutations of the two elements
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interacting in three-body configurations. Thus for 3 elements, 27
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entries would be required, etc.</p>
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<p>As annotated above, the first element in the entry is the center atom
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in a three-body interaction. Thus an entry for SiCC means a Si atom
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with 2 C atoms as neighbors. The parameter values used for the
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two-body interaction come from the entry where the 2nd and 3rd
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elements are the same. Thus the two-body parameters for Si
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interacting with C, comes from the SiCC entry. The three-body
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parameters can in principle be specific to the three elements of the
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configuration. In the literature, however, the three-body parameters
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are usually defined by simple formulas involving two sets of pair-wise
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parameters, corresponding to the ij and ik pairs, where i is the
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center atom. The user must ensure that the correct combining rule is
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used to calculate the values of the threebody parameters for
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alloys. Note also that the function phi3 contains two exponential
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screening factors with parameter values from the ij pair and ik
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pairs. So phi3 for a C atom bonded to a Si atom and a second C atom
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will depend on the three-body parameters for the CSiC entry, and also
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on the two-body parameters for the CCC and CSiSi entries. Since the
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order of the two neighbors is arbitrary, the threebody parameters for
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entries CSiC and CCSi should be the same. Similarly, the two-body
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parameters for entries SiCC and CSiSi should also be the same. The
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parameters used only for two-body interactions (A, B, p, and q) in
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entries whose 2nd and 3rd element are different (e.g. SiCSi) are not
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used for anything and can be set to 0.0 if desired.
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This is also true for the parameters in phi3 that are
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taken from the ij and ik pairs (sigma, a, gamma)</p>
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<hr class="docutils" />
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<p>Styles with a <em>cuda</em>, <em>gpu</em>, <em>intel</em>, <em>kk</em>, <em>omp</em>, or <em>opt</em> suffix are
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functionally the same as the corresponding style without the suffix.
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They have been optimized to run faster, depending on your available
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hardware, as discussed in <a class="reference internal" href="Section_accelerate.html"><em>Section_accelerate</em></a>
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of the manual. The accelerated styles take the same arguments and
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should produce the same results, except for round-off and precision
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issues.</p>
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<p>These accelerated styles are part of the USER-CUDA, GPU, USER-INTEL,
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KOKKOS, USER-OMP and OPT packages, respectively. They are only
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enabled if LAMMPS was built with those packages. See the <a class="reference internal" href="Section_start.html#start-3"><span>Making LAMMPS</span></a> section for more info.</p>
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<p>You can specify the accelerated styles explicitly in your input script
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by including their suffix, or you can use the <a class="reference internal" href="Section_start.html#start-7"><span>-suffix command-line switch</span></a> when you invoke LAMMPS, or you can
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use the <a class="reference internal" href="suffix.html"><em>suffix</em></a> command in your input script.</p>
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<p>When using the USER-INTEL package with this style, there is an
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additional 5 to 10 percent performance improvement when the
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Stillinger-Weber parameters p and q are set to 4 and 0 respectively.
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These parameters are common for modeling silicon and water.</p>
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<p>See <a class="reference internal" href="Section_accelerate.html"><em>Section_accelerate</em></a> of the manual for
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more instructions on how to use the accelerated styles effectively.</p>
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<hr class="docutils" />
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<p><strong>Mixing, shift, table, tail correction, restart, rRESPA info</strong>:</p>
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<p>For atom type pairs I,J and I != J, where types I and J correspond to
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two different element types, mixing is performed by LAMMPS as
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described above from values in the potential file.</p>
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<p>This pair style does not support the <a class="reference internal" href="pair_modify.html"><em>pair_modify</em></a>
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shift, table, and tail options.</p>
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<p>This pair style does not write its information to <a class="reference internal" href="restart.html"><em>binary restart files</em></a>, since it is stored in potential files. Thus, you
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need to re-specify the pair_style and pair_coeff commands in an input
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script that reads a restart file.</p>
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<p>This pair style can only be used via the <em>pair</em> keyword of the
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<a class="reference internal" href="run_style.html"><em>run_style respa</em></a> command. It does not support the
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<em>inner</em>, <em>middle</em>, <em>outer</em> keywords.</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 pair style is part of the MANYBODY package. It is only enabled
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if LAMMPS was built with that package (which it is by default). See
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the <a class="reference internal" href="Section_start.html#start-3"><span>Making LAMMPS</span></a> section for more info.</p>
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<p>This pair style requires the <a class="reference internal" href="newton.html"><em>newton</em></a> setting to be “on”
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for pair interactions.</p>
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<p>The Stillinger-Weber potential files provided with LAMMPS (see the
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potentials directory) are parameterized for metal <a class="reference internal" href="units.html"><em>units</em></a>.
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You can use the SW potential with any LAMMPS units, but you would need
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to create your own SW potential file with coefficients listed in the
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appropriate units if your simulation doesn’t use “metal” units.</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_coeff.html"><em>pair_coeff</em></a></p>
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<p><strong>Default:</strong> none</p>
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<hr class="docutils" />
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<p id="stillinger"><strong>(Stillinger)</strong> Stillinger and Weber, Phys Rev B, 31, 5262 (1985).</p>
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</div>
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