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<title>pair_style tersoff/mod command — LAMMPS 15 May 2015 version documentation</title>
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<div class="section" id="pair-style-tersoff-mod-command">
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<span id="index-0"></span><h1>pair_style tersoff/mod command<a class="headerlink" href="#pair-style-tersoff-mod-command" title="Permalink to this headline">¶</a></h1>
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</div>
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<div class="section" id="pair-style-tersoff-mod-kk-command">
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<h1>pair_style tersoff/mod/kk command<a class="headerlink" href="#pair-style-tersoff-mod-kk-command" title="Permalink to this headline">¶</a></h1>
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</div>
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<div class="section" id="pair-style-tersoff-mod-omp-command">
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<h1>pair_style tersoff/mod/omp command<a class="headerlink" href="#pair-style-tersoff-mod-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 tersoff/mod
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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<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 tersoff/mod
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pair_coeff * * Si.tersoff.mod Si Si
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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>tersoff/mod</em> style computes a bond-order type interatomic
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potential <a class="reference internal" href="#kumagai"><span>(Kumagai)</span></a> based on a 3-body Tersoff potential
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<a class="reference internal" href="pair_tersoff_zbl.html#tersoff-1"><span>(Tersoff_1)</span></a>, <a class="reference internal" href="pair_tersoff_zbl.html#tersoff-2"><span>(Tersoff_2)</span></a> with modified
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cutoff function and angular-dependent term, giving the energy E of a
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system of atoms as</p>
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<img alt="_images/pair_tersoff_mod.jpg" class="align-center" src="_images/pair_tersoff_mod.jpg" />
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<p>where f_R is a two-body term and f_A includes three-body interactions.
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The summations in the formula are over all neighbors J and K of atom I
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within a cutoff distance = R + D.</p>
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<p>The modified cutoff function f_C proposed by <a class="reference internal" href="#murty"><span>(Murty)</span></a> and
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having a continuous second-order differential is employed. The
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angular-dependent term g(theta) was modified to increase the
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flexibility of the potential.</p>
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<p>The <em>tersoff/mod</em> potential is fitted to both the elastic constants
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and melting point by employing the modified Tersoff potential function
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form in which the angular-dependent term is improved. The model
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performs extremely well in describing the crystalline, liquid, and
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amorphous phases <a class="reference internal" href="#schelling"><span>(Schelling)</span></a>.</p>
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<p>Only a single pair_coeff command is used with the <em>tersoff/mod</em> style
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which specifies a Tersoff/MOD potential file with parameters for all
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needed elements. These are mapped to LAMMPS atom types by specifying
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N 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 Tersoff/MOD elements to atom types</li>
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</ul>
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<p>As an example, imagine the Si.tersoff_mod file has Tersoff values for Si.
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If your LAMMPS simulation has 3 Si atoms types, you would use the following
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pair_coeff command:</p>
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<div class="highlight-python"><div class="highlight"><pre>pair_coeff * * Si.tersoff_mod Si Si Si
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</pre></div>
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</div>
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<p>The 1st 2 arguments must be * * so as to span all LAMMPS atom types.
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The three Si arguments map LAMMPS atom types 1,2,3 to the Si element
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in the Tersoff/MOD file. If a mapping value is specified as NULL, the
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mapping is not performed. This can be used when a <em>tersoff/mod</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>Tersoff/MOD file in the <em>potentials</em> directory of the LAMMPS
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distribution have a ”.tersoff.mod” suffix. Lines that are not blank
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or comments (starting with #) define parameters for a triplet of
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elements. The parameters in a single entry correspond to coefficients
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in the formula 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 (the atom bonded to the center atom)</li>
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<li>element 3 (the atom influencing the 1-2 bond in a bond-order sense)</li>
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<li>beta</li>
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<li>alpha</li>
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<li>h</li>
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<li>eta</li>
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<li>beta_ters = 1 (dummy parameter)</li>
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<li>lambda2 (1/distance units)</li>
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<li>B (energy units)</li>
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<li>R (distance units)</li>
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<li>D (distance units)</li>
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<li>lambda1 (1/distance units)</li>
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<li>A (energy units)</li>
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<li>n</li>
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<li>c1</li>
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<li>c2</li>
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<li>c3</li>
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<li>c4</li>
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<li>c5</li>
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</ul>
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<p>The n, eta, lambda2, B, lambda1, and A parameters are only used for
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two-body interactions. The beta, alpha, c1, c2, c3, c4, c5, h
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parameters are only used for three-body interactions. The R and D
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parameters are used for both two-body and three-body interactions. The
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non-annotated parameters are unitless.</p>
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<p>The Tersoff/MOD potential file must contain entries for all the elements
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listed in the pair_coeff command. It can also contain entries for
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additional elements not being used in a particular simulation; LAMMPS
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ignores those entries.</p>
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<p>For a single-element simulation, only a single entry is required
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(e.g. SiSiSi). As annotated above, the first element in the entry is
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the center atom in a three-body interaction and it is bonded to the
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2nd atom and the bond is influenced by the 3rd atom. Thus an entry
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for SiSiSi means Si bonded to a Si with another Si atom influencing the bond.</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 style does not support the <a class="reference internal" href="pair_modify.html"><em>pair_modify</em></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"><em>binary restart files</em></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"><em>run_style respa</em></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<a class="headerlink" href="#restrictions" title="Permalink to this headline">¶</a></h2>
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<p>This pair style is part of the MANYBODY package. It is only enabled
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if LAMMPS was built with that package (which it is by default). See
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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>This pair style requires the <a class="reference internal" href="newton.html"><em>newton</em></a> setting to be “on”
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for pair interactions.</p>
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<p>The Tersoff/MOD potential files provided with LAMMPS (see the potentials
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directory) are parameterized for metal <a class="reference internal" href="units.html"><em>units</em></a>. You can
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use the Tersoff/MOD potential with any LAMMPS units, but you would need to
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create your own Tersoff/MOD 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<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="kumagai"><strong>(Kumagai)</strong> T. Kumagai, S. Izumi, S. Hara, S. Sakai,
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Comp. Mat. Science, 39, 457 (2007).</p>
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<p id="tersoff-1"><strong>(Tersoff_1)</strong> J. Tersoff, Phys Rev B, 37, 6991 (1988).</p>
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<p id="tersoff-2"><strong>(Tersoff_2)</strong> J. Tersoff, Phys Rev B, 38, 9902 (1988).</p>
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<p id="murty"><strong>(Murty)</strong> M.V.R. Murty, H.A. Atwater, Phys Rev B, 51, 4889 (1995).</p>
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<p id="schelling"><strong>(Schelling)</strong> Patrick K. Schelling, Comp. Mat. Science, 44, 274 (2008).</p>
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