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<div class="section" id="pair-style-nb3b-harmonic-command">
<span id="index-0"></span><h1>pair_style nb3b/harmonic command</h1>
</div>
<div class="section" id="pair-style-nb3b-harmonic-omp-command">
<h1>pair_style nb3b/harmonic/omp command</h1>
<div class="section" id="syntax">
<h2>Syntax</h2>
<div class="highlight-default"><div class="highlight"><pre><span></span><span class="n">pair_style</span> <span class="n">nb3b</span><span class="o">/</span><span class="n">harmonic</span>
</pre></div>
</div>
</div>
<div class="section" id="examples">
<h2>Examples</h2>
<div class="highlight-default"><div class="highlight"><pre><span></span><span class="n">pair_style</span> <span class="n">nb3b</span><span class="o">/</span><span class="n">harmonic</span>
<span class="n">pair_coeff</span> <span class="o">*</span> <span class="o">*</span> <span class="n">MgOH</span><span class="o">.</span><span class="n">nb3bharmonic</span> <span class="n">Mg</span> <span class="n">O</span> <span class="n">H</span>
</pre></div>
</div>
</div>
<div class="section" id="description">
<h2>Description</h2>
<p>This pair style computes a nonbonded 3-body harmonic potential for the
energy E of a system of atoms as</p>
<img alt="_images/pair_nb3b_harmonic.jpg" class="align-center" src="_images/pair_nb3b_harmonic.jpg" />
<p>where <em>theta_0</em> is the equilibrium value of the angle and <em>K</em> is a
prefactor. Note that the usual 1/2 factor is included in <em>K</em>. The form
of the potential is identical to that used in angle_style <em>harmonic</em>,
but in this case, the atoms do not need to be explicitly bonded.</p>
<p>Only a single pair_coeff command is used with this style which
specifies a potential file with parameters for specified elements.
These are mapped to LAMMPS atom types by specifying N additional
arguments after the filename in the pair_coeff command, where N is the
number of LAMMPS atom types:</p>
<ul class="simple">
<li>filename</li>
<li>N element names = mapping of elements to atom types</li>
</ul>
<p>See the <a class="reference internal" href="pair_coeff.html"><span class="doc">pair_coeff</span></a> doc page for alternate ways
to specify the path for the potential file.</p>
<p>As an example, imagine a file SiC.nb3b.harmonic has potential values
for Si and C. If your LAMMPS simulation has 4 atoms types and you
want the 1st 3 to be Si, and the 4th to be C, you would use the
following pair_coeff command:</p>
<div class="highlight-default"><div class="highlight"><pre><span></span><span class="n">pair_coeff</span> <span class="o">*</span> <span class="o">*</span> <span class="n">SiC</span><span class="o">.</span><span class="n">nb3b</span><span class="o">.</span><span class="n">harmonic</span> <span class="n">Si</span> <span class="n">Si</span> <span class="n">Si</span> <span class="n">C</span>
</pre></div>
</div>
<p>The 1st 2 arguments must be * * so as to span all LAMMPS atom types.
The first three Si arguments map LAMMPS atom types 1,2,3 to the Si
element in the potential file. The final C argument maps LAMMPS atom
type 4 to the C element in the potential file. If a mapping value is
specified as NULL, the mapping is not performed. This can be used
when the potential is used as part of the <em>hybrid</em> pair style. The
NULL values are placeholders for atom types that will be used with
other potentials. An example of a pair_coeff command for use with the
<em>hybrid</em> pair style is:</p>
<p>pair_coeff * * nb3b/harmonic MgOH.nb3b.harmonic Mg O H</p>
<p>Three-body nonbonded harmonic files in the <em>potentials</em> directory of
the LAMMPS distribution have a &#8221;.nb3b.harmonic&#8221; suffix. Lines that
are not blank or comments (starting with #) define parameters for a
triplet of elements.</p>
<p>Each entry has six arguments. The first three are atom types as
referenced in the LAMMPS input file. The first argument specifies the
central atom. The fourth argument indicates the <em>K</em> parameter. The
fifth argument indicates <em>theta_0</em>. The sixth argument indicates a
separation cutoff in Angstroms.</p>
<p>For a given entry, if the second and third arguments are identical,
then the entry is for a cutoff for the distance between types 1 and 2
(values for <em>K</em> and <em>theta_0</em> are irrelevant in this case).</p>
<p>For a given entry, if the first three arguments are all different,
then the entry is for the <em>K</em> and <em>theta_0</em> parameters (the cutoff in
this case is irrelevant).</p>
<p>It is <em>not</em> required that the potential file contain entries for all
of the elements listed in the pair_coeff command. It can also contain
entries for additional elements not being used in a particular
simulation; LAMMPS ignores those entries.</p>
<hr class="docutils" />
<p>Styles with a <em>gpu</em>, <em>intel</em>, <em>kk</em>, <em>omp</em>, or <em>opt</em> suffix are
functionally the same as the corresponding style without the suffix.
They have been optimized to run faster, depending on your available
hardware, as discussed in <a class="reference internal" href="Section_accelerate.html"><span class="doc">Section_accelerate</span></a>
of the manual. The accelerated styles take the same arguments and
should produce the same results, except for round-off and precision
issues.</p>
<p>These accelerated styles are part of the GPU, USER-INTEL, KOKKOS,
USER-OMP and OPT packages, respectively. They are only enabled if
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>
<p>You can specify the accelerated styles explicitly in your input script
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
use the <a class="reference internal" href="suffix.html"><span class="doc">suffix</span></a> command in your input script.</p>
<p>See <a class="reference internal" href="Section_accelerate.html"><span class="doc">Section_accelerate</span></a> of the manual for
more instructions on how to use the accelerated styles effectively.</p>
</div>
<hr class="docutils" />
<div class="section" id="restrictions">
<h2>Restrictions</h2>
<p>This pair style can only be used if LAMMPS was built with the MANYBODY
package (which it is by default). 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 on packages.</p>
</div>
<div class="section" id="related-commands">
<h2>Related commands</h2>
<p><a class="reference internal" href="pair_coeff.html"><span class="doc">pair_coeff</span></a></p>
<p><strong>Default:</strong> none</p>
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