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<div class="section" id="pair-style-buck-long-coul-long-command">
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<span id="index-0"></span><h1>pair_style buck/long/coul/long command<a class="headerlink" href="#pair-style-buck-long-coul-long-command" title="Permalink to this headline">¶</a></h1>
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<div class="section" id="pair-style-buck-long-coul-long-omp-command">
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<h1>pair_style buck/long/coul/long/omp command<a class="headerlink" href="#pair-style-buck-long-coul-long-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 buck/long/coul/long flag_buck flag_coul cutoff (cutoff2)
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</pre></div>
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</div>
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<ul class="simple">
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<li>flag_buck = <em>long</em> or <em>cut</em></li>
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</ul>
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<pre class="literal-block">
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<em>long</em> = use Kspace long-range summation for the dispersion term 1/r^6
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<em>cut</em> = use a cutoff
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</pre>
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<ul class="simple">
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<li>flag_coul = <em>long</em> or <em>off</em></li>
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</ul>
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<pre class="literal-block">
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<em>long</em> = use Kspace long-range summation for the Coulombic term 1/r
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<em>off</em> = omit the Coulombic term
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</pre>
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<ul class="simple">
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<li>cutoff = global cutoff for Buckingham (and Coulombic if only 1 cutoff) (distance units)</li>
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<li>cutoff2 = global cutoff for Coulombic (optional) (distance units)</li>
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</ul>
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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 buck/long/coul/long cut off 2.5
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pair_style buck/long/coul/long cut long 2.5 4.0
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pair_style buck/long/coul/long long long 4.0
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pair_coeff * * 1 1
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pair_coeff 1 1 1 3 4
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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>buck/long/coul/long</em> style computes a Buckingham potential (exp/6
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instead of Lennard-Jones 12/6) and Coulombic potential, given by</p>
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<img alt="_images/pair_buck.jpg" class="align-center" src="_images/pair_buck.jpg" />
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<img alt="_images/pair_coulomb.jpg" class="align-center" src="_images/pair_coulomb.jpg" />
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<p>Rc is the cutoff. If one cutoff is specified in the pair_style
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command, it is used for both the Buckingham and Coulombic terms. If
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two cutoffs are specified, they are used as cutoffs for the Buckingham
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and Coulombic terms respectively.</p>
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<p>The purpose of this pair style is to capture long-range interactions
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resulting from both attractive 1/r^6 Buckingham and Coulombic 1/r
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interactions. This is done by use of the <em>flag_buck</em> and <em>flag_coul</em>
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settings. The “<a class="reference internal" href="#ismail"><span>Ismail</span></a> paper has more details on when it is
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appropriate to include long-range 1/r^6 interactions, using this
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potential.</p>
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<p>If <em>flag_buck</em> is set to <em>long</em>, no cutoff is used on the Buckingham
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1/r^6 dispersion term. The long-range portion can be calculated by
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using the <a class="reference internal" href="kspace_style.html"><em>kspace_style ewald/disp or pppm/disp</em></a>
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commands. The specified Buckingham cutoff then determines which
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portion of the Buckingham interactions are computed directly by the
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pair potential versus which part is computed in reciprocal space via
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the Kspace style. If <em>flag_buck</em> is set to <em>cut</em>, the Buckingham
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interactions are simply cutoff, as with <a class="reference internal" href="pair_buck.html"><em>pair_style buck</em></a>.</p>
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<p>If <em>flag_coul</em> is set to <em>long</em>, no cutoff is used on the Coulombic
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interactions. The long-range portion can calculated by using any of
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several <a class="reference internal" href="kspace_style.html"><em>kspace_style</em></a> command options such as
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<em>pppm</em> or <em>ewald</em>. Note that if <em>flag_buck</em> is also set to long, then
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the <em>ewald/disp</em> or <em>pppm/disp</em> Kspace style needs to be used to
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perform the long-range calculations for both the Buckingham and
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Coulombic interactions. If <em>flag_coul</em> is set to <em>off</em>, Coulombic
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interactions are not computed.</p>
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<p>The following coefficients must be defined for each pair of atoms
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types via the <a class="reference internal" href="pair_coeff.html"><em>pair_coeff</em></a> command as in the examples
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above, or in the data file or restart files read by the
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<a class="reference internal" href="read_data.html"><em>read_data</em></a> or <a class="reference internal" href="read_restart.html"><em>read_restart</em></a>
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commands:</p>
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<ul class="simple">
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<li>A (energy units)</li>
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<li>rho (distance units)</li>
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<li>C (energy-distance^6 units)</li>
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<li>cutoff (distance units)</li>
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<li>cutoff2 (distance units)</li>
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</ul>
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<p>The second coefficient, rho, must be greater than zero.</p>
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<p>The latter 2 coefficients are optional. If not specified, the global
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Buckingham and Coulombic cutoffs specified in the pair_style command
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are used. If only one cutoff is specified, it is used as the cutoff
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for both Buckingham and Coulombic interactions for this type pair. If
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both coefficients are specified, they are used as the Buckingham and
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Coulombic cutoffs for this type pair. Note that if you are using
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<em>flag_buck</em> set to <em>long</em>, you cannot specify a Buckingham cutoff for
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an atom type pair, since only one global Buckingham cutoff is allowed.
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Similarly, if you are using <em>flag_coul</em> set to <em>long</em>, you cannot
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specify a Coulombic cutoff for an atom type pair, since only one
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global Coulombic cutoff is allowed.</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>This pair styles does not support mixing. Thus, coefficients for all
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I,J pairs must be specified explicitly.</p>
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<p>This pair style supports the <a class="reference internal" href="pair_modify.html"><em>pair_modify</em></a> shift
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option for the energy of the exp() and 1/r^6 portion of the pair
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interaction, assuming <em>flag_buck</em> is <em>cut</em>.</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 option for the energy of the Buckingham portion of the pair
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interaction.</p>
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<p>This pair style supports the <a class="reference internal" href="pair_modify.html"><em>pair_modify</em></a> table and
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table/disp options since they can tabulate the short-range portion of
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the long-range Coulombic and dispersion interactions.</p>
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<p>This pair style write its information to <a class="reference internal" href="restart.html"><em>binary restart files</em></a>, so pair_style and pair_coeff commands do not need
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to be specified in an input script that reads a restart file.</p>
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<p>This pair style supports the use of the <em>inner</em>, <em>middle</em>, and <em>outer</em>
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keywords of the <a class="reference internal" href="run_style.html"><em>run_style respa</em></a> command, meaning the
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pairwise forces can be partitioned by distance at different levels of
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the rRESPA hierarchy. See the <a class="reference internal" href="run_style.html"><em>run_style</em></a> command for
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details.</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 style is part of the KSPACE package. It is only enabled if
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LAMMPS was built with that package. See the <a class="reference internal" href="Section_start.html#start-3"><span>Making LAMMPS</span></a> section for more info. Note that
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the KSPACE package is installed by default.</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="ismail"><strong>(Ismail)</strong> Ismail, Tsige, In ‘t Veld, Grest, Molecular Physics
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(accepted) (2007).</p>
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