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<div class="section" id="fix-press-berendsen-command">
<span id="index-0"></span><h1>fix press/berendsen command<a class="headerlink" href="#fix-press-berendsen-command" title="Permalink to this headline">¶</a></h1>
<div class="section" id="syntax">
<h2>Syntax<a class="headerlink" href="#syntax" title="Permalink to this headline">¶</a></h2>
<div class="highlight-python"><div class="highlight"><pre>fix ID group-ID press/berendsen keyword value ...
</pre></div>
</div>
<ul class="simple">
<li>ID, group-ID are documented in <a class="reference internal" href="fix.html"><em>fix</em></a> command</li>
<li>press/berendsen = style name of this fix command</li>
</ul>
<pre class="literal-block">
one or more keyword value pairs may be appended
keyword = <em>iso</em> or <em>aniso</em> or <em>x</em> or <em>y</em> or <em>z</em> or <em>couple</em> or <em>dilate</em> or <em>modulus</em>
<em>iso</em> or <em>aniso</em> values = Pstart Pstop Pdamp
Pstart,Pstop = scalar external pressure at start/end of run (pressure units)
Pdamp = pressure damping parameter (time units)
<em>x</em> or <em>y</em> or <em>z</em> values = Pstart Pstop Pdamp
Pstart,Pstop = external stress tensor component at start/end of run (pressure units)
Pdamp = stress damping parameter (time units)
<em>couple</em> = <em>none</em> or <em>xyz</em> or <em>xy</em> or <em>yz</em> or <em>xz</em>
<em>modulus</em> value = bulk modulus of system (pressure units)
<em>dilate</em> value = <em>all</em> or <em>partial</em>
</pre>
</div>
<div class="section" id="examples">
<h2>Examples<a class="headerlink" href="#examples" title="Permalink to this headline">¶</a></h2>
<div class="highlight-python"><div class="highlight"><pre>fix 1 all press/berendsen iso 0.0 0.0 1000.0
fix 2 all press/berendsen aniso 0.0 0.0 1000.0 dilate partial
</pre></div>
</div>
</div>
<div class="section" id="description">
<h2>Description<a class="headerlink" href="#description" title="Permalink to this headline">¶</a></h2>
<p>Reset the pressure of the system by using a Berendsen barostat
<a class="reference internal" href="fix_temp_berendsen.html#berendsen"><span>(Berendsen)</span></a>, which rescales the system volume and
(optionally) the atoms coordinates within the simulation box every
timestep.</p>
<p>Regardless of what atoms are in the fix group, a global pressure is
computed for all atoms. Similarly, when the size of the simulation
box is changed, all atoms are re-scaled to new positions, unless the
keyword <em>dilate</em> is specified with a value of <em>partial</em>, in which case
only the atoms in the fix group are re-scaled. The latter can be
useful for leaving the coordinates of atoms in a solid substrate
unchanged and controlling the pressure of a surrounding fluid.</p>
<div class="admonition note">
<p class="first admonition-title">Note</p>
<p class="last">Unlike the <a class="reference internal" href="fix_nh.html"><em>fix npt</em></a> or <a class="reference internal" href="fix_nh.html"><em>fix nph</em></a>
commands which perform Nose/Hoover barostatting AND time integration,
this fix does NOT perform time integration. It only modifies the box
size and atom coordinates to effect barostatting. Thus you must use a
separate time integration fix, like <a class="reference internal" href="fix_nve.html"><em>fix nve</em></a> or <a class="reference internal" href="fix_nh.html"><em>fix nvt</em></a> to actually update the positions and velocities of
atoms. This fix can be used in conjunction with thermostatting fixes
to control the temperature, such as <a class="reference internal" href="fix_nh.html"><em>fix nvt</em></a> or <a class="reference internal" href="fix_langevin.html"><em>fix langevin</em></a> or <a class="reference internal" href="fix_temp_berendsen.html"><em>fix temp/berendsen</em></a>.</p>
</div>
<p>See <a class="reference internal" href="Section_howto.html#howto-16"><span>this howto section</span></a> of the manual for
a discussion of different ways to compute temperature and perform
thermostatting and barostatting.</p>
<hr class="docutils" />
<p>The barostat is specified using one or more of the <em>iso</em>, <em>aniso</em>,
<em>x</em>, <em>y</em>, <em>z</em>, and <em>couple</em> keywords. These keywords give you the
ability to specify the 3 diagonal components of an external stress
tensor, and to couple various of these components together so that the
dimensions they represent are varied together during a
constant-pressure simulation. Unlike the <a class="reference internal" href="fix_nh.html"><em>fix npt</em></a> and
<a class="reference internal" href="fix_nh.html"><em>fix nph</em></a> commands, this fix cannot be used with triclinic
(non-orthogonal) simulation boxes to control all 6 components of the
general pressure tensor.</p>
<p>The target pressures for each of the 3 diagonal components of the
stress tensor can be specified independently via the <em>x</em>, <em>y</em>, <em>z</em>,
keywords, which correspond to the 3 simulation box dimensions. For
each component, the external pressure or tensor component at each
timestep is a ramped value during the run from <em>Pstart</em> to <em>Pstop</em>.
If a target pressure is specified for a component, then the
corresponding box dimension will change during a simulation. For
example, if the <em>y</em> keyword is used, the y-box length will change. A
box dimension will not change if that component is not specified,
although you have the option to change that dimension via the <a class="reference internal" href="fix_deform.html"><em>fix deform</em></a> command.</p>
<p>For all barostat keywords, the <em>Pdamp</em> parameter determines the time
scale on which pressure is relaxed. For example, a value of 10.0
means to relax the pressure in a timespan of (roughly) 10 time units
(tau or fmsec or psec - see the <a class="reference internal" href="units.html"><em>units</em></a> command).</p>
<div class="admonition note">
<p class="first admonition-title">Note</p>
<p class="last">A Berendsen barostat will not work well for arbitrary values of
<em>Pdamp</em>. If <em>Pdamp</em> is too small, the pressure and volume can
fluctuate wildly; if it is too large, the pressure will take a very
long time to equilibrate. A good choice for many models is a <em>Pdamp</em>
of around 1000 timesteps. However, note that <em>Pdamp</em> is specified in
time units, and that timesteps are NOT the same as time units for most
<a class="reference internal" href="units.html"><em>units</em></a> settings.</p>
</div>
<div class="admonition note">
<p class="first admonition-title">Note</p>
<p class="last">The relaxation time is actually also a function of the bulk
modulus of the system (inverse of isothermal compressibility). The
bulk modulus has units of pressure and is the amount of pressure that
would need to be applied (isotropically) to reduce the volume of the
system by a factor of 2 (assuming the bulk modulus was a constant,
independent of density, which it&#8217;s not). The bulk modulus can be set
via the keyword <em>modulus</em>. The <em>Pdamp</em> parameter is effectively
multiplied by the bulk modulus, so if the pressure is relaxing faster
than expected or desired, increasing the bulk modulus has the same
effect as increasing <em>Pdamp</em>. The converse is also true. LAMMPS does
not attempt to guess a correct value of the bulk modulus; it just uses
10.0 as a default value which gives reasonable relaxation for a
Lennard-Jones liquid, but will be way off for other materials and way
too small for solids. Thus you should experiment to find appropriate
values of <em>Pdamp</em> and/or the <em>modulus</em> when using this fix.</p>
</div>
<hr class="docutils" />
<p>The <em>couple</em> keyword allows two or three of the diagonal components of
the pressure tensor to be &#8220;coupled&#8221; together. The value specified
with the keyword determines which are coupled. For example, <em>xz</em>
means the <em>Pxx</em> and <em>Pzz</em> components of the stress tensor are coupled.
<em>Xyz</em> means all 3 diagonal components are coupled. Coupling means two
things: the instantaneous stress will be computed as an average of the
corresponding diagonal components, and the coupled box dimensions will
be changed together in lockstep, meaning coupled dimensions will be
dilated or contracted by the same percentage every timestep. The
<em>Pstart</em>, <em>Pstop</em>, <em>Pdamp</em> parameters for any coupled dimensions must
be identical. <em>Couple xyz</em> can be used for a 2d simulation; the <em>z</em>
dimension is simply ignored.</p>
<hr class="docutils" />
<p>The <em>iso</em> and <em>aniso</em> keywords are simply shortcuts that are
equivalent to specifying several other keywords together.</p>
<p>The keyword <em>iso</em> means couple all 3 diagonal components together when
pressure is computed (hydrostatic pressure), and dilate/contract the
dimensions together. Using &#8220;iso Pstart Pstop Pdamp&#8221; is the same as
specifying these 4 keywords:</p>
<div class="highlight-python"><div class="highlight"><pre>x Pstart Pstop Pdamp
y Pstart Pstop Pdamp
z Pstart Pstop Pdamp
couple xyz
</pre></div>
</div>
<p>The keyword <em>aniso</em> means <em>x</em>, <em>y</em>, and <em>z</em> dimensions are controlled
independently using the <em>Pxx</em>, <em>Pyy</em>, and <em>Pzz</em> components of the
stress tensor as the driving forces, and the specified scalar external
pressure. Using &#8220;aniso Pstart Pstop Pdamp&#8221; is the same as specifying
these 4 keywords:</p>
<div class="highlight-python"><div class="highlight"><pre>x Pstart Pstop Pdamp
y Pstart Pstop Pdamp
z Pstart Pstop Pdamp
couple none
</pre></div>
</div>
<hr class="docutils" />
<p>This fix computes a temperature and pressure each timestep. To do
this, the fix creates its own computes of style &#8220;temp&#8221; and &#8220;pressure&#8221;,
as if these commands had been issued:</p>
<div class="highlight-python"><div class="highlight"><pre>compute fix-ID_temp group-ID temp
compute fix-ID_press group-ID pressure fix-ID_temp
</pre></div>
</div>
<p>See the <a class="reference internal" href="compute_temp.html"><em>compute temp</em></a> and <a class="reference internal" href="compute_pressure.html"><em>compute pressure</em></a> commands for details. Note that the
IDs of the new computes are the fix-ID + underscore + &#8220;temp&#8221; or fix_ID
+ underscore + &#8220;press&#8221;, and the group for the new computes is the same
as the fix group.</p>
<p>Note that these are NOT the computes used by thermodynamic output (see
the <a class="reference internal" href="thermo_style.html"><em>thermo_style</em></a> command) with ID = <em>thermo_temp</em>
and <em>thermo_press</em>. This means you can change the attributes of this
fix&#8217;s temperature or pressure via the
<a class="reference internal" href="compute_modify.html"><em>compute_modify</em></a> command or print this temperature
or pressure during thermodynamic output via the <a class="reference internal" href="thermo_style.html"><em>thermo_style custom</em></a> command using the appropriate compute-ID.
It also means that changing attributes of <em>thermo_temp</em> or
<em>thermo_press</em> will have no effect on this fix.</p>
</div>
<div class="section" id="restart-fix-modify-output-run-start-stop-minimize-info">
<h2>Restart, fix_modify, output, run start/stop, minimize info<a class="headerlink" href="#restart-fix-modify-output-run-start-stop-minimize-info" title="Permalink to this headline">¶</a></h2>
<p>No information about this fix is written to <a class="reference internal" href="restart.html"><em>binary restart files</em></a>.</p>
<p>The <a class="reference internal" href="fix_modify.html"><em>fix_modify</em></a> <em>temp</em> and <em>press</em> options are
supported by this fix. You can use them to assign a
<a class="reference internal" href="compute.html"><em>compute</em></a> you have defined to this fix which will be used
in its temperature and pressure calculations. If you do this, note
that the kinetic energy derived from the compute temperature should be
consistent with the virial term computed using all atoms for the
pressure. LAMMPS will warn you if you choose to compute temperature
on a subset of atoms.</p>
<p>No global or per-atom quantities are stored by this fix for access by
various <a class="reference internal" href="Section_howto.html#howto-15"><span>output commands</span></a>.</p>
<p>This fix can ramp its target pressure over multiple runs, using the
<em>start</em> and <em>stop</em> keywords of the <a class="reference internal" href="run.html"><em>run</em></a> command. See the
<a class="reference internal" href="run.html"><em>run</em></a> command for details of how to do this.</p>
<p>This fix is not invoked during <a class="reference internal" href="minimize.html"><em>energy minimization</em></a>.</p>
</div>
<div class="section" id="restrictions">
<h2>Restrictions<a class="headerlink" href="#restrictions" title="Permalink to this headline">¶</a></h2>
<p>Any dimension being adjusted by this fix must be periodic.</p>
</div>
<div class="section" id="related-commands">
<h2>Related commands<a class="headerlink" href="#related-commands" title="Permalink to this headline">¶</a></h2>
<p><a class="reference internal" href="fix_nve.html"><em>fix nve</em></a>, <a class="reference internal" href="fix_nh.html"><em>fix nph</em></a>, <a class="reference internal" href="fix_nh.html"><em>fix npt</em></a>, <a class="reference internal" href="fix_temp_berendsen.html"><em>fix temp/berendsen</em></a>,
<a class="reference internal" href="fix_modify.html"><em>fix_modify</em></a></p>
</div>
<div class="section" id="default">
<h2>Default<a class="headerlink" href="#default" title="Permalink to this headline">¶</a></h2>
<p>The keyword defaults are dilate = all, modulus = 10.0 in units of
pressure for whatever <a class="reference internal" href="units.html"><em>units</em></a> are defined.</p>
<hr class="docutils" />
<p id="berendsen"><strong>(Berendsen)</strong> Berendsen, Postma, van Gunsteren, DiNola, Haak, J Chem
Phys, 81, 3684 (1984).</p>
</div>
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