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<li class="toctree-l1"><a class="reference internal" href="Section_intro.html">1. Introduction</a></li>
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<li>improper_style class2 command</li>
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<div class="section" id="improper-style-class2-command">
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<span id="index-0"></span><h1>improper_style class2 command</h1>
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</div>
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<div class="section" id="improper-style-class2-omp-command">
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<h1>improper_style class2/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">improper_style</span> <span class="n">class2</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">improper_style</span> <span class="n">class2</span>
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<span class="n">improper_coeff</span> <span class="mi">1</span> <span class="mf">100.0</span> <span class="mi">0</span>
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<span class="n">improper_coeff</span> <span class="o">*</span> <span class="n">aa</span> <span class="mf">0.0</span> <span class="mf">0.0</span> <span class="mf">0.0</span> <span class="mf">115.06</span> <span class="mf">130.01</span> <span class="mf">115.06</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>class2</em> improper style uses the potential</p>
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<img alt="_images/improper_class2.jpg" class="align-center" src="_images/improper_class2.jpg" />
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<p>where Ei is the improper term and Eaa is an angle-angle term. The 3 X
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terms in Ei are an average over 3 out-of-plane angles.</p>
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<p>The 4 atoms in an improper quadruplet (listed in the data file read by
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the <a class="reference internal" href="read_data.html"><span class="doc">read_data</span></a> command) are ordered I,J,K,L. X_IJKL
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refers to the angle between the plane of I,J,K and the plane of J,K,L,
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and the bond JK lies in both planes. Similarly for X_KJLI and X_LJIK.
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Note that atom J appears in the common bonds (JI, JK, JL) of all 3 X
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terms. Thus J (the 2nd atom in the quadruplet) is the atom of
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symmetry in the 3 X angles.</p>
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<p>The subscripts on the various theta’s refer to different combinations
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of 3 atoms (I,J,K,L) used to form a particular angle. E.g. Theta_IJL
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is the angle formed by atoms I,J,L with J in the middle. Theta1,
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theta2, theta3 are the equilibrium positions of those angles. Again,
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atom J (the 2nd atom in the quadruplet) is the atom of symmetry in the
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theta angles, since it is always the center atom.</p>
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<p>Since atom J is the atom of symmetry, normally the bonds J-I, J-K, J-L
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would exist for an improper to be defined between the 4 atoms, but
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this is not required.</p>
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<p>See <a class="reference internal" href="#improper-sun"><span class="std std-ref">(Sun)</span></a> for a description of the COMPASS class2 force field.</p>
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<p>Coefficients for the Ei and Eaa formulas must be defined for each
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improper type via the <a class="reference internal" href="improper_coeff.html"><span class="doc">improper_coeff</span></a> command as
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in the example above, or in the data file or restart files read by the
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<a class="reference internal" href="read_data.html"><span class="doc">read_data</span></a> or <a class="reference internal" href="read_restart.html"><span class="doc">read_restart</span></a>
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commands.</p>
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<p>These are the 2 coefficients for the Ei formula:</p>
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<ul class="simple">
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<li>K (energy/radian^2)</li>
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<li>X0 (degrees)</li>
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</ul>
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<p>X0 is specified in degrees, but LAMMPS converts it to radians
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internally; hence the units of K are in energy/radian^2.</p>
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<p>For the Eaa formula, each line in a
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<a class="reference internal" href="improper_coeff.html"><span class="doc">improper_coeff</span></a> command in the input script lists
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7 coefficients, the first of which is “aa” to indicate they are
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AngleAngle coefficients. In a data file, these coefficients should be
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listed under a “AngleAngle Coeffs” heading and you must leave out the
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“aa”, i.e. only list 6 coefficients after the improper type.</p>
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<ul class="simple">
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<li>aa</li>
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<li>M1 (energy/distance)</li>
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<li>M2 (energy/distance)</li>
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<li>M3 (energy/distance)</li>
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<li>theta1 (degrees)</li>
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<li>theta2 (degrees)</li>
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<li>theta3 (degrees)</li>
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</ul>
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<p>The theta values are specified in degrees, but LAMMPS converts them to
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radians internally; hence the units of M are in energy/radian^2.</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"><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 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 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 improper style can only be used if LAMMPS was built with the
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CLASS2 package. See the <a class="reference internal" href="Section_start.html#start-3"><span class="std std-ref">Making LAMMPS</span></a>
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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="improper_coeff.html"><span class="doc">improper_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="improper-sun"><strong>(Sun)</strong> Sun, J Phys Chem B 102, 7338-7364 (1998).</p>
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