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<div class="section" id="compute-pressure-command">
<span id="index-0"></span><h1>compute pressure command</h1>
<div class="section" id="syntax">
<h2>Syntax</h2>
<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>
</pre></div>
</div>
<ul class="simple">
<li>ID, group-ID are documented in <a class="reference internal" href="compute.html"><span class="doc">compute</span></a> command</li>
<li>pressure = style name of this compute command</li>
<li>temp-ID = ID of compute that calculates temperature, can be NULL if not needed</li>
<li>zero or more keywords may be appended</li>
<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>
</ul>
</div>
<div class="section" id="examples">
<h2>Examples</h2>
<pre class="literal-block">
compute 1 all pressure thermo_temp
compute 1 all pressure NULL pair bond
</pre>
</div>
<div class="section" id="description">
<h2>Description</h2>
<p>Define a computation that calculates the pressure of the entire system
of atoms. The specified group must be &#8220;all&#8221;. 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
pressure (stress). These per-atom values could be summed for a group
of atoms via the <a class="reference internal" href="compute_reduce.html"><span class="doc">compute reduce</span></a> command.</p>
<p>The pressure is computed by the formula</p>
<img alt="_images/pressure.jpg" class="align-center" src="_images/pressure.jpg" />
<p>where N is the number of atoms in the system (see discussion of DOF
below), Kb is the Boltzmann constant, T is the temperature, d is the
dimensionality of the system (2 or 3 for 2d/3d), V is the system
volume (or area in 2d), and the second term is the virial, computed
within LAMMPS for all pairwise as well as 2-body, 3-body, and 4-body,
and long-range interactions. <a class="reference internal" href="fix.html"><span class="doc">Fixes</span></a> that impose constraints
(e.g. the <a class="reference internal" href="fix_shake.html"><span class="doc">fix shake</span></a> command) also contribute to the
virial term.</p>
<p>A symmetric pressure tensor, stored as a 6-element vector, is also
calculated by this compute. The 6 components of the vector are
ordered xx, yy, zz, xy, xz, yz. The equation for the I,J components
(where I and J = x,y,z) is similar to the above formula, except that
the first term uses components of the kinetic energy tensor and the
second term uses components of the virial tensor:</p>
<img alt="_images/pressure_tensor.jpg" class="align-center" src="_images/pressure_tensor.jpg" />
<p>If no extra keywords are listed, the entire equations above are
calculated. This includes a kinetic energy (temperature) term and the
virial as the sum of pair, bond, angle, dihedral, improper, kspace
(long-range), and fix contributions to the force on each atom. If any
extra keywords are listed, then only those components are summed to
compute temperature or ke and/or the virial. The <em>virial</em> keyword
means include all terms except the kinetic energy <em>ke</em>.</p>
<p>Details of how LAMMPS computes the virial efficiently for the entire
system, including the effects of periodic boundary conditions is
discussed in <a class="reference internal" href="compute_stress_atom.html#thompson"><span class="std std-ref">(Thompson)</span></a>.</p>
<p>The temperature and kinetic energy tensor is not calculated by this
compute, but rather by the temperature compute specified with the
command. If the kinetic energy is not included in the pressure, than
the temperature compute is not used and can be specified as NULL.
Normally the temperature compute used by compute pressure should
calculate the temperature of all atoms for consistency with the virial
term, but any compute style that calculates temperature can be used,
e.g. one that excludes frozen atoms or other degrees of freedom.</p>
<p>Note that if desired the specified temperature compute can be one that
subtracts off a bias to calculate a temperature using only the thermal
velocity of the atoms, e.g. by subtracting a background streaming
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>
<p>Also note that the N in the first formula above is really
degrees-of-freedom divided by d = dimensionality, where the DOF value
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>
<p>A compute of this style with the ID of &#8220;thermo_press&#8221; is created when
LAMMPS starts up, as if this command were in the input script:</p>
<pre class="literal-block">
compute thermo_press all pressure thermo_temp
</pre>
<p>where &#8220;thermo_temp&#8221; is the ID of a similarly defined compute of style
&#8220;temp&#8221;. See the &#8220;thermo_style&#8221; command for more details.</p>
<hr class="docutils" />
<p>Styles with a <em>gpu</em>, <em>intel</em>, <em>kk</em>, <em>omp</em>, or <em>opt</em> suffix are
functionally the same as the corresponding style without the suffix.
They have been optimized to run faster, depending on your available
hardware, as discussed in <a class="reference internal" href="Section_accelerate.html"><span class="doc">Section_accelerate</span></a>
of the manual. The accelerated styles take the same arguments and
should produce the same results, except for round-off and precision
issues.</p>
<p>These accelerated styles are part of the GPU, USER-INTEL, KOKKOS,
USER-OMP and OPT packages, respectively. They are only 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>
<p>You can specify the accelerated styles explicitly in your input script
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
use the <a class="reference internal" href="suffix.html"><span class="doc">suffix</span></a> command in your input script.</p>
<p>See <a class="reference internal" href="Section_accelerate.html"><span class="doc">Section_accelerate</span></a> of the manual for
more instructions on how to use the accelerated styles effectively.</p>
<hr class="docutils" />
<p><strong>Output info:</strong></p>
<p>This compute calculates a global scalar (the pressure) and a global
vector of length 6 (pressure tensor), which can be accessed by indices
1-6. These values can be used by any command that uses global scalar
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
options.</p>
<p>The scalar and vector values calculated by this compute are
&#8220;intensive&#8221;. The scalar and vector values will be in pressure
<a class="reference internal" href="units.html"><span class="doc">units</span></a>.</p>
</div>
<div class="section" id="restrictions">
<h2>Restrictions</h2>
<blockquote>
<div>none</div></blockquote>
</div>
<div class="section" id="related-commands">
<h2>Related commands</h2>
<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>,
<a class="reference internal" href="thermo_style.html"><span class="doc">thermo_style</span></a>,</p>
<p><strong>Default:</strong> none</p>
<hr class="docutils" />
<p id="thompson"><strong>(Thompson)</strong> Thompson, Plimpton, Mattson, J Chem Phys, 131, 154107 (2009).</p>
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