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<div class="section" id="dihedral-style-charmm-command">
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<span id="index-0"></span><h1>dihedral_style charmm command</h1>
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</div>
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<div class="section" id="dihedral-style-charmm-intel-command">
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<h1>dihedral_style charmm/intel command</h1>
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</div>
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<div class="section" id="dihedral-style-charmm-kk-command">
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<h1>dihedral_style charmm/kk command</h1>
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</div>
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<div class="section" id="dihedral-style-charmm-omp-command">
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<h1>dihedral_style charmm/omp command</h1>
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<div class="section" id="syntax">
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<h2>Syntax</h2>
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<div class="highlight-default"><div class="highlight"><pre><span></span><span class="n">dihedral_style</span> <span class="n">charmm</span>
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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</h2>
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<div class="highlight-default"><div class="highlight"><pre><span></span><span class="n">dihedral_style</span> <span class="n">charmm</span>
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<span class="n">dihedral_coeff</span> <span class="mi">1</span> <span class="mf">0.2</span> <span class="mi">1</span> <span class="mi">180</span> <span class="mf">1.0</span>
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<span class="n">dihedral_coeff</span> <span class="mi">2</span> <span class="mf">1.8</span> <span class="mi">1</span> <span class="mi">0</span> <span class="mf">1.0</span>
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<span class="n">dihedral_coeff</span> <span class="mi">1</span> <span class="mf">3.1</span> <span class="mi">2</span> <span class="mi">180</span> <span class="mf">0.5</span>
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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</h2>
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<p>The <em>charmm</em> dihedral style uses the potential</p>
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<img alt="_images/dihedral_charmm.jpg" class="align-center" src="_images/dihedral_charmm.jpg" />
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<p>See <a class="reference internal" href="#dihedral-mackerell"><span class="std std-ref">(MacKerell)</span></a> for a description of the CHARMM
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force field. This dihedral style can also be used for the AMBER force
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field (see comment on weighting factors below). See
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<a class="reference internal" href="#dihedral-cornell"><span class="std std-ref">(Cornell)</span></a> for a description of the AMBER force
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field.</p>
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<p>The following coefficients must be defined for each dihedral type via the
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<a class="reference internal" href="dihedral_coeff.html"><span class="doc">dihedral_coeff</span></a> command as in the example above, or in
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the data file or restart files read by the <a class="reference internal" href="read_data.html"><span class="doc">read_data</span></a>
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or <a class="reference internal" href="read_restart.html"><span class="doc">read_restart</span></a> commands:</p>
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<ul class="simple">
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<li>K (energy)</li>
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<li>n (integer >= 0)</li>
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<li>d (integer value of degrees)</li>
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<li>weighting factor (1.0, 0.5, or 0.0)</li>
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</ul>
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<p>The weighting factor is required to correct for double counting
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pairwise non-bonded Lennard-Jones interactions in cyclic systems or
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when using the CHARMM dihedral style with non-CHARMM force fields.
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With the CHARMM dihedral style, interactions between the 1st and 4th
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atoms in a dihedral are skipped during the normal non-bonded force
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computation and instead evaluated as part of the dihedral using
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special epsilon and sigma values specified with the
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<a class="reference internal" href="pair_charmm.html"><span class="doc">pair_coeff</span></a> command of pair styles that contain
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“lj/charmm” (e.g. <a class="reference internal" href="pair_charmm.html"><span class="doc">pair_style lj/charmm/coul/long</span></a>)
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In 6-membered rings, the same 1-4 interaction would be computed twice
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(once for the clockwise 1-4 pair in dihedral 1-2-3-4 and once in the
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counterclockwise dihedral 1-6-5-4) and thus the weighting factor has
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to be 0.5 in this case. In 4-membered or 5-membered rings, the 1-4
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dihedral also is counted as a 1-2 or 1-3 interaction when going around
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the ring in the opposite direction and thus the weighting factor is
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0.0, as the 1-2 and 1-3 exclusions take precedence.</p>
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<p>Note that this dihedral weighting factor is unrelated to the scaling
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factor specified by the <a class="reference internal" href="special_bonds.html"><span class="doc">special bonds</span></a> command
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which applies to all 1-4 interactions in the system. For CHARMM force
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fields, the special_bonds 1-4 interaction scaling factor should be set
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to 0.0. Since the corresponding 1-4 non-bonded interactions are
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computed with the dihedral. This means that if any of the weighting
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factors defined as dihedral coefficients (4th coeff above) are
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non-zero, then you must use a pair style with “lj/charmm” and set the
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special_bonds 1-4 scaling factor to 0.0 (which is the
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default). Otherwise 1-4 non-bonded interactions in dihedrals will be
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computed twice.</p>
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<p>Also note that for AMBER force fields, which use pair styles with
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“lj/cut”, the special_bonds 1-4 scaling factor should be set to the
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AMBER defaults (1/2 and 5/6) and all the dihedral weighting factors
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(4th coeff above) must be set to 0.0. In this case, you can use any
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pair style you wish, since the dihedral does not need any
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Lennard-Jones parameter information and will not compute any 1-4
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non-bonded interactions.</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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</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>This dihedral style can only be used if LAMMPS was built with the
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MOLECULE 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>
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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="dihedral_coeff.html"><span class="doc">dihedral_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="dihedral-cornell"><strong>(Cornell)</strong> Cornell, Cieplak, Bayly, Gould, Merz, Ferguson,
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Spellmeyer, Fox, Caldwell, Kollman, JACS 117, 5179-5197 (1995).</p>
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<p id="dihedral-mackerell"><strong>(MacKerell)</strong> MacKerell, Bashford, Bellott, Dunbrack, Evanseck, Field,
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Fischer, Gao, Guo, Ha, et al, J Phys Chem B, 102, 3586 (1998).</p>
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