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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>compute pressure command</li>
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<div class="section" id="compute-pressure-command">
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<span id="index-0"></span><h1>compute pressure 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">compute</span> <span class="n">ID</span> <span class="n">group</span><span class="o">-</span><span class="n">ID</span> <span class="n">pressure</span> <span class="n">temp</span><span class="o">-</span><span class="n">ID</span> <span class="n">keyword</span> <span class="o">...</span>
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</pre></div>
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</div>
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<ul class="simple">
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<li>ID, group-ID are documented in <a class="reference internal" href="compute.html"><span class="doc">compute</span></a> command</li>
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<li>pressure = style name of this compute command</li>
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<li>temp-ID = ID of compute that calculates temperature, can be NULL if not needed</li>
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<li>zero or more keywords may be appended</li>
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<li>keyword = <em>ke</em> or <em>pair</em> or <em>bond</em> or <em>angle</em> or <em>dihedral</em> or <em>improper</em> or <em>kspace</em> or <em>fix</em> or <em>virial</em></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</h2>
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<div class="highlight-default"><div class="highlight"><pre><span></span><span class="n">compute</span> <span class="mi">1</span> <span class="nb">all</span> <span class="n">pressure</span> <span class="n">thermo_temp</span>
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<span class="n">compute</span> <span class="mi">1</span> <span class="nb">all</span> <span class="n">pressure</span> <span class="n">NULL</span> <span class="n">pair</span> <span class="n">bond</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>Define a computation that calculates the pressure of the entire system
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of atoms. The specified group must be “all”. See the <a class="reference internal" href="compute_stress_atom.html"><span class="doc">compute stress/atom</span></a> command if you want per-atom
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pressure (stress). These per-atom values could be summed for a group
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of atoms via the <a class="reference internal" href="compute_reduce.html"><span class="doc">compute reduce</span></a> command.</p>
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<p>The pressure is computed by the formula</p>
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<img alt="_images/pressure.jpg" class="align-center" src="_images/pressure.jpg" />
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<p>where N is the number of atoms in the system (see discussion of DOF
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below), Kb is the Boltzmann constant, T is the temperature, d is the
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dimensionality of the system (2 or 3 for 2d/3d), V is the system
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volume (or area in 2d), and the second term is the virial, computed
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within LAMMPS for all pairwise as well as 2-body, 3-body, and 4-body,
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and long-range interactions. <a class="reference internal" href="fix.html"><span class="doc">Fixes</span></a> that impose constraints
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(e.g. the <a class="reference internal" href="fix_shake.html"><span class="doc">fix shake</span></a> command) also contribute to the
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virial term.</p>
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<p>A symmetric pressure tensor, stored as a 6-element vector, is also
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calculated by this compute. The 6 components of the vector are
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ordered xx, yy, zz, xy, xz, yz. The equation for the I,J components
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(where I and J = x,y,z) is similar to the above formula, except that
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the first term uses components of the kinetic energy tensor and the
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second term uses components of the virial tensor:</p>
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<img alt="_images/pressure_tensor.jpg" class="align-center" src="_images/pressure_tensor.jpg" />
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<p>If no extra keywords are listed, the entire equations above are
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calculated. This includes a kinetic energy (temperature) term and the
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virial as the sum of pair, bond, angle, dihedral, improper, kspace
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(long-range), and fix contributions to the force on each atom. If any
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extra keywords are listed, then only those components are summed to
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compute temperature or ke and/or the virial. The <em>virial</em> keyword
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means include all terms except the kinetic energy <em>ke</em>.</p>
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<p>Details of how LAMMPS computes the virial efficiently for the entire
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system, including the effects of periodic boundary conditions is
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discussed in <a class="reference internal" href="compute_stress_atom.html#thompson"><span class="std std-ref">(Thompson)</span></a>.</p>
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<p>The temperature and kinetic energy tensor is not calculated by this
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compute, but rather by the temperature compute specified with the
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command. If the kinetic energy is not included in the pressure, than
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the temperature compute is not used and can be specified as NULL.
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Normally the temperature compute used by compute pressure should
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calculate the temperature of all atoms for consistency with the virial
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term, but any compute style that calculates temperature can be used,
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e.g. one that excludes frozen atoms or other degrees of freedom.</p>
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<p>Note that if desired the specified temperature compute can be one that
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subtracts off a bias to calculate a temperature using only the thermal
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velocity of the atoms, e.g. by subtracting a background streaming
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velocity. See the doc pages for individual <a class="reference internal" href="compute.html"><span class="doc">compute commands</span></a> to determine which ones include a bias.</p>
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<p>Also note that the N in the first formula above is really
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degrees-of-freedom divided by d = dimensionality, where the DOF value
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is calcluated by the temperature compute. See the various <a class="reference internal" href="compute.html"><span class="doc">compute temperature</span></a> styles for details.</p>
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<p>A compute of this style with the ID of “thermo_press” is created when
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LAMMPS starts up, as if this command were in the input script:</p>
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<div class="highlight-default"><div class="highlight"><pre><span></span><span class="n">compute</span> <span class="n">thermo_press</span> <span class="nb">all</span> <span class="n">pressure</span> <span class="n">thermo_temp</span>
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</pre></div>
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</div>
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<p>where “thermo_temp” is the ID of a similarly defined compute of style
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“temp”. See the “thermo_style” command for more details.</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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<hr class="docutils" />
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<p><strong>Output info:</strong></p>
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<p>This compute calculates a global scalar (the pressure) and a global
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vector of length 6 (pressure tensor), which can be accessed by indices
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1-6. These values can be used by any command that uses global scalar
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or vector values from a compute as input. See <a class="reference internal" href="Section_howto.html#howto-15"><span class="std std-ref">this section</span></a> for an overview of LAMMPS output
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options.</p>
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<p>The scalar and vector values calculated by this compute are
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“intensive”. The scalar and vector values will be in pressure
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<a class="reference internal" href="units.html"><span class="doc">units</span></a>.</p>
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</div>
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<div class="section" id="restrictions">
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<h2>Restrictions</h2>
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<blockquote>
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<div>none</div></blockquote>
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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="compute_temp.html"><span class="doc">compute temp</span></a>, <a class="reference internal" href="compute_stress_atom.html"><span class="doc">compute stress/atom</span></a>,
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<a class="reference internal" href="thermo_style.html"><span class="doc">thermo_style</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="thompson"><strong>(Thompson)</strong> Thompson, Plimpton, Mattson, J Chem Phys, 131, 154107 (2009).</p>
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