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<li>pair_style vashishta command</li>
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<div class="section" id="pair-style-vashishta-command">
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<span id="index-0"></span><h1>pair_style vashishta command</h1>
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</div>
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<div class="section" id="pair-style-vashishta-omp-command">
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<h1>pair_style vashishta/omp command</h1>
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</div>
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<div class="section" id="pair-style-vashishta-table-command">
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<h1>pair_style vashishta/table command</h1>
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</div>
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<div class="section" id="pair-style-vashishta-table-omp-command">
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<h1>pair_style vashishta/table/omp command</h1>
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<div class="section" id="syntax">
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<h2>Syntax</h2>
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<pre class="literal-block">
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pair_style style args
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</pre>
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<ul class="simple">
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<li>style = <em>vashishta</em> or <em>vashishta/table</em> or <em>vashishta/omp</em> or <em>vashishta/table/omp</em></li>
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<li>args = list of arguments for a particular style</li>
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</ul>
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<pre class="literal-block">
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<em>vashishta</em> or <em>vashishta/omp</em> args = none
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<em>vashishta/table</em> or <em>vashishta/table/omp</em> args = Ntable cutinner
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Ntable = # of tabulation points
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cutinner = tablulate from cutinner to cutoff
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</pre>
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</div>
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<div class="section" id="examples">
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<h2>Examples</h2>
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<pre class="literal-block">
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pair_style vashishta
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pair_coeff * * SiC.vashishta Si C
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</pre>
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<pre class="literal-block">
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pair_style vashishta/table 100000 0.2
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pair_coeff * * SiC.vashishta Si C
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</pre>
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</div>
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<div class="section" id="description">
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<h2>Description</h2>
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<p>The <em>vashishta</em> and <em>vashishta/table</em> styles compute the combined
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2-body and 3-body family of potentials developed in the group of Priya
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Vashishta and collaborators. By combining repulsive, screened
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Coulombic, screened charge-dipole, and dispersion interactions with a
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bond-angle energy based on the Stillinger-Weber potential, this
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potential has been used to describe a variety of inorganic compounds,
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including SiO2 <a class="reference internal" href="#vashishta1990"><span class="std std-ref">Vashishta1990</span></a>, SiC
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<a class="reference internal" href="#vashishta2007"><span class="std std-ref">Vashishta2007</span></a>, and InP <a class="reference internal" href="#branicio2009"><span class="std std-ref">Branicio2009</span></a>.</p>
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<p>The potential for the energy U of a system of atoms is</p>
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<img alt="_images/pair_vashishta.jpg" class="align-center" src="_images/pair_vashishta.jpg" />
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<p>where we follow the notation used in <a class="reference internal" href="#branicio2009"><span class="std std-ref">Branicio2009</span></a>.
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U2 is a two-body term and U3 is a three-body term. The
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summation over two-body terms is over all neighbors J within
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a cutoff distance = <em>rc</em>. The twobody terms are shifted and
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tilted by a linear function so that the energy and force are
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both zero at <em>rc</em>. The summation over three-body terms
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is over all neighbors J and K within a cut-off distance = <em>r0</em>,
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where the exponential screening function becomes zero.</p>
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<p>The <em>vashishta</em> style computes these formulas analytically. The
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<em>vashishta/table</em> style tabulates the analytic values for <em>Ntable</em>
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points from cutinner to the cutoff of the potential. The points are
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equally spaced in R^2 space from cutinner^2 to cutoff^2. For the
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two-body term in the above equation, a linear interpolation for each
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pairwise distance between adjacent points in the table. In practice
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the tabulated version can run 3-5x faster than the analytic version
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with with moderate to little loss of accuracy for Ntable values
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between 10000 and 1000000. It is not recommended to use less than
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5000 tabulation points.</p>
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<p>Only a single pair_coeff command is used with either style which
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specifies a Vashishta potential file with parameters for all needed
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elements. These are mapped to LAMMPS atom types by specifying N
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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 Vashishta 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"><span class="doc">pair_coeff</span></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.vashishta has parameters 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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<pre class="literal-block">
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pair_coeff * * SiC.vashishta Si Si Si C
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</pre>
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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 file. The final C argument maps LAMMPS atom type 4
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to the C element in the 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>vashishta</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>Vashishta files in the <em>potentials</em> directory of the LAMMPS
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distribution have a ”.vashishta” 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 formulae 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>H (energy units)</li>
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<li>eta</li>
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<li>Zi (electron charge units)</li>
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<li>Zj (electron charge units)</li>
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<li>lambda1 (distance units)</li>
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<li>D (energy units)</li>
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<li>lambda4 (distance units)</li>
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<li>W (energy units)</li>
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<li>rc (distance units)</li>
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<li>B (energy units)</li>
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<li>gamma</li>
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<li>r0 (distance units)</li>
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<li>C</li>
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<li>costheta0</li>
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</ul>
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<p>The non-annotated parameters are unitless. The Vashishta potential
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file must contain entries for all the elements listed in the
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pair_coeff command. It can also contain entries for additional
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elements not being used in a particular simulation; LAMMPS ignores
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those entries. For a single-element simulation, only a single entry
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is required (e.g. SiSiSi). For a two-element simulation, the file
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must contain 8 entries (for SiSiSi, SiSiC, SiCSi, SiCC, CSiSi, CSiC,
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CCSi, CCC), that specify parameters for all permutations of the two
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elements interacting in three-body configurations. Thus for 3
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elements, 27 entries would be required, etc.</p>
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<p>Depending on the particular version of the Vashishta potential, the
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values of these parameters may be keyed to the identities of zero,
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one, two, or three elements. In order to make the input file format
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unambiguous, general, and simple to code, LAMMPS uses a slightly
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confusing method for specifying parameters. All parameters are
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divided into two classes: two-body and three-body. Two-body and
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three-body parameters are handled differently, as described below.
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The two-body parameters are H, eta, lambda1, D, lambda4, W, rc, gamma,
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and r0. They appear in the above formulae with two subscripts. The
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parameters Zi and Zj are also classified as two-body parameters, even
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though they only have 1 subscript. The three-body parameters are B,
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C, costheta0. They appear in the above formulae with three
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subscripts. Two-body and three-body parameters are handled
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differently, as described below.</p>
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<p>The first element in each entry is the center atom in a three-body
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interaction, while the second and third elements are two neighbor
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atoms. Three-body parameters for a central atom I and two neighbors J
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and K are taken from the IJK entry. Note that even though three-body
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parameters do not depend on the order of J and K, LAMMPS stores
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three-body parameters for both IJK and IKJ. The user must ensure that
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these values are equal. Two-body parameters for an atom I interacting
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with atom J are taken from the IJJ entry, where the 2nd and 3rd
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elements are the same. Thus the two-body parameters for Si interacting
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with C come from the SiCC entry. Note that even though two-body
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parameters (except possibly gamma and r0 in U3) do not depend on the
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order of the two elements, LAMMPS will get the Si-C value from the
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SiCC entry and the C-Si value from the CSiSi entry. The user must
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ensure that these values are equal. Two-body parameters appearing in
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entries where the 2nd and 3rd elements are different are stored but
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never used. It is good practice to enter zero for these values. Note
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that the three-body function U3 above contains the two-body parameters
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gamma and r0. So U3 for a central C atom bonded to an Si atom and a
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second C atom will take three-body parameters from the CSiC entry, but
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two-body parameters from the CCC and CSiSi entries.</p>
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<hr class="docutils" />
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<p>Styles with a <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"><span class="doc">Section_accelerate</span></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 GPU, USER-INTEL, KOKKOS,
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USER-OMP and OPT packages, respectively. They are only enabled if
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LAMMPS was built with those packages. See the <a class="reference internal" href="Section_start.html#start-3"><span class="std std-ref">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 class="std std-ref">-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"><span class="doc">suffix</span></a> command in your input script.</p>
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<p>See <a class="reference internal" href="Section_accelerate.html"><span class="doc">Section_accelerate</span></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"><span class="doc">pair_modify</span></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"><span class="doc">binary restart files</span></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"><span class="doc">run_style respa</span></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</h2>
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<p>These pair style are part of the MANYBODY package. They is only
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enabled if LAMMPS was built with that package (which it is by
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default). See the <a class="reference internal" href="Section_start.html#start-3"><span class="std std-ref">Making LAMMPS</span></a> section
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for more info.</p>
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<p>These pair styles requires the <a class="reference internal" href="newton.html"><span class="doc">newton</span></a> setting to be “on”
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for pair interactions.</p>
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<p>The Vashishta potential files provided with LAMMPS (see the potentials
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directory) are parameterized for metal <a class="reference internal" href="units.html"><span class="doc">units</span></a>. You can
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use the Vashishta potential with any LAMMPS units, but you would need
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to create your own 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</h2>
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<p><a class="reference internal" href="pair_coeff.html"><span class="doc">pair_coeff</span></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="vashishta1990"><strong>(Vashishta1990)</strong> P. Vashishta, R. K. Kalia, J. P. Rino, Phys. Rev. B
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41, 12197 (1990).</p>
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<p id="vashishta2007"><strong>(Vashishta2007)</strong> P. Vashishta, R. K. Kalia, A. Nakano,
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J. P. Rino. J. Appl. Phys. 101, 103515 (2007).</p>
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<p id="branicio2009"><strong>(Branicio2009)</strong> Branicio, Rino, Gan and Tsuzuki, J. Phys Condensed
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Matter 21 (2009) 095002</p>
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