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<title>pair_style tersoff command — LAMMPS 15 May 2015 version documentation</title>
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<div class="section" id="pair-style-tersoff-command">
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<span id="index-0"></span><h1>pair_style tersoff command<a class="headerlink" href="#pair-style-tersoff-command" title="Permalink to this headline">¶</a></h1>
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</div>
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<div class="section" id="pair-style-tersoff-table-command">
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<h1>pair_style tersoff/table command<a class="headerlink" href="#pair-style-tersoff-table-command" title="Permalink to this headline">¶</a></h1>
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</div>
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<div class="section" id="pair-style-tersoff-cuda">
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<h1>pair_style tersoff/cuda<a class="headerlink" href="#pair-style-tersoff-cuda" title="Permalink to this headline">¶</a></h1>
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</div>
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<div class="section" id="pair-style-tersoff-gpu">
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<h1>pair_style tersoff/gpu<a class="headerlink" href="#pair-style-tersoff-gpu" title="Permalink to this headline">¶</a></h1>
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</div>
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<div class="section" id="pair-style-tersoff-kk">
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<h1>pair_style tersoff/kk<a class="headerlink" href="#pair-style-tersoff-kk" title="Permalink to this headline">¶</a></h1>
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</div>
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<div class="section" id="pair-style-tersoff-omp">
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<h1>pair_style tersoff/omp<a class="headerlink" href="#pair-style-tersoff-omp" title="Permalink to this headline">¶</a></h1>
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</div>
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<div class="section" id="pair-style-tersoff-table-omp-command">
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<h1>pair_style tersoff/table/omp command<a class="headerlink" href="#pair-style-tersoff-table-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 style
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</pre></div>
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</div>
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<p>style = <em>tersoff</em> or <em>tersoff/table</em> or <em>tersoff/cuda</em> or <em>tersoff/gpu</em> or <em>tersoff/omp</em> or <em>tersoff/table/omp</em></p>
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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 tersoff
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pair_coeff * * Si.tersoff Si
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pair_coeff * * SiC.tersoff Si C Si
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</pre></div>
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</div>
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<div class="highlight-python"><div class="highlight"><pre>pair_style tersoff/table
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pair_coeff * * SiCGe.tersoff Si(D)
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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>tersoff</em> style computes a 3-body Tersoff potential
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<a class="reference internal" href="#tersoff-1"><span>(Tersoff_1)</span></a> for the energy E of a system of atoms as</p>
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<img alt="_images/pair_tersoff_1.jpg" class="align-center" src="_images/pair_tersoff_1.jpg" />
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<p>where f_R is a two-body term and f_A includes three-body interactions.
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The summations in the formula are over all neighbors J and K of atom I
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within a cutoff distance = R + D.</p>
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<p>The <em>tersoff/table</em> style uses tabulated forms for the two-body,
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environment and angular functions. Linear interpolation is performed
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between adjacent table entries. The table length is chosen to be
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accurate within 10^-6 with respect to the <em>tersoff</em> style energy.
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The <em>tersoff/table</em> should give better performance in terms of speed.</p>
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<p>Only a single pair_coeff command is used with the <em>tersoff</em> style
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which specifies a Tersoff 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 Tersoff 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 the SiC.tersoff file has Tersoff values for Si
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and C. If your LAMMPS simulation has 4 atoms types and you want the
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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.tersoff 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 Tersoff file. The final C argument maps LAMMPS atom
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type 4 to the C element in the Tersoff file. If a mapping value is
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specified as NULL, the mapping is not performed. This can be used
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when a <em>tersoff</em> potential is used as part of the <em>hybrid</em> pair style.
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The NULL values are placeholders for atom types that will be used with
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other potentials.</p>
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<p>Tersoff files in the <em>potentials</em> directory of the LAMMPS distribution
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have a ”.tersoff” suffix. Lines that are not blank or comments
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(starting with #) define parameters for a triplet of elements. The
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parameters in a single entry correspond to coefficients in the formula
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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 (the atom bonded to the center atom)</li>
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<li>element 3 (the atom influencing the 1-2 bond in a bond-order sense)</li>
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<li>m</li>
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<li>gamma</li>
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<li>lambda3 (1/distance units)</li>
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<li>c</li>
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<li>d</li>
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<li>costheta0 (can be a value < -1 or > 1)</li>
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<li>n</li>
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<li>beta</li>
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<li>lambda2 (1/distance units)</li>
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<li>B (energy units)</li>
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<li>R (distance units)</li>
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<li>D (distance units)</li>
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<li>lambda1 (1/distance units)</li>
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<li>A (energy units)</li>
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</ul>
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<p>The n, beta, lambda2, B, lambda1, and A parameters are only used for
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two-body interactions. The m, gamma, lambda3, c, d, and costheta0
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parameters are only used for three-body interactions. The R and D
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parameters are used for both two-body and three-body interactions. The
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non-annotated parameters are unitless. The value of m must be 3 or 1.</p>
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<p>The Tersoff potential file must contain entries for all the elements
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listed in the pair_coeff command. It can also contain entries for
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additional elements not being used in a particular simulation; LAMMPS
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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 Tersoff 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 and it is bonded to the 2nd atom and the
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bond is influenced by the 3rd atom. Thus an entry for SiCC means Si
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bonded to a C with another C atom influencing the bond. Thus
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three-body parameters for SiCSi and SiSiC entries will not, in
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general, be the same. The parameters used for the two-body
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interaction come from the entry where the 2nd element is repeated.
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Thus the two-body parameters for Si interacting with C, comes from the
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SiCC entry.</p>
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<p>The parameters used for a particular
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three-body interaction come from the entry with the corresponding
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three elements. The parameters used only for two-body interactions
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(n, beta, lambda2, B, lambda1, and A) in entries whose 2nd and 3rd
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element are different (e.g. SiCSi) are not used for anything and can
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be set to 0.0 if desired.</p>
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<p>Note that the twobody parameters in entries such as SiCC and CSiSi
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are often the same, due to the common use of symmetric mixing rules,
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but this is not always the case. For example, the beta and n parameters in
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Tersoff_2 <a class="reference internal" href="#tersoff-2"><span>(Tersoff_2)</span></a> are not symmetric.</p>
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<p>We chose the above form so as to enable users to define all commonly
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used variants of the Tersoff potential. In particular, our form
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reduces to the original Tersoff form when m = 3 and gamma = 1, while
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it reduces to the form of <a class="reference internal" href="#albe"><span>Albe et al.</span></a> when beta = 1 and m = 1.
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Note that in the current Tersoff implementation in LAMMPS, m must be
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specified as either 3 or 1. Tersoff used a slightly different but
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equivalent form for alloys, which we will refer to as Tersoff_2
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potential <a class="reference internal" href="#tersoff-2"><span>(Tersoff_2)</span></a>. The <em>tersoff/table</em> style implements
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Tersoff_2 parameterization only.</p>
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<p>LAMMPS parameter values for Tersoff_2 can be obtained as follows:
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gamma_ijk = omega_ik, lambda3 = 0 and the value of
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m has no effect. The parameters for species i and j can be calculated
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using the Tersoff_2 mixing rules:</p>
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<img alt="_images/pair_tersoff_2.jpg" class="align-center" src="_images/pair_tersoff_2.jpg" />
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<p>Tersoff_2 parameters R and S must be converted to the LAMMPS
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parameters R and D (R is different in both forms), using the following
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relations: R=(R’+S’)/2 and D=(S’-R’)/2, where the primes indicate the
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Tersoff_2 parameters.</p>
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<p>In the potentials directory, the file SiCGe.tersoff provides the
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LAMMPS parameters for Tersoff’s various versions of Si, as well as his
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alloy parameters for Si, C, and Ge. This file can be used for pure Si,
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(three different versions), pure C, pure Ge, binary SiC, and binary
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SiGe. LAMMPS will generate an error if this file is used with any
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combination involving C and Ge, since there are no entries for the GeC
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interactions (Tersoff did not publish parameters for this
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cross-interaction.) Tersoff files are also provided for the SiC alloy
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(SiC.tersoff) and the GaN (GaN.tersoff) alloys.</p>
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<p>Many thanks to Rutuparna Narulkar, David Farrell, and Xiaowang Zhou
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for helping clarify how Tersoff parameters for alloys have been
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defined in various papers.</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>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 Tersoff potential files provided with LAMMPS (see the potentials
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directory) are parameterized for metal <a class="reference internal" href="units.html"><em>units</em></a>. You can
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use the Tersoff potential with any LAMMPS units, but you would need to
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create your own Tersoff 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="tersoff-1"><strong>(Tersoff_1)</strong> J. Tersoff, Phys Rev B, 37, 6991 (1988).</p>
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<p id="albe"><strong>(Albe)</strong> J. Nord, K. Albe, P. Erhart, and K. Nordlund, J. Phys.:
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Condens. Matter, 15, 5649(2003).</p>
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<p id="tersoff-2"><strong>(Tersoff_2)</strong> J. Tersoff, Phys Rev B, 39, 5566 (1989); errata (PRB 41, 3248)</p>
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