<p>Reset the pressure of the system by using a Berendsen barostat
<aclass="reference internal"href="fix_temp_berendsen.html#berendsen"><spanclass="std std-ref">(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>
<divclass="admonition note">
<pclass="first admonition-title">Note</p>
<pclass="last">Unlike the <aclass="reference internal"href="fix_nh.html"><spanclass="doc">fix npt</span></a> or <aclass="reference internal"href="fix_nh.html"><spanclass="doc">fix nph</span></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 <aclass="reference internal"href="fix_nve.html"><spanclass="doc">fix nve</span></a> or <aclass="reference internal"href="fix_nh.html"><spanclass="doc">fix nvt</span></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 <aclass="reference internal"href="fix_nh.html"><spanclass="doc">fix nvt</span></a> or <aclass="reference internal"href="fix_langevin.html"><spanclass="doc">fix langevin</span></a> or <aclass="reference internal"href="fix_temp_berendsen.html"><spanclass="doc">fix temp/berendsen</span></a>.</p>
</div>
<p>See <aclass="reference internal"href="Section_howto.html#howto-16"><spanclass="std std-ref">this howto section</span></a> of the manual for
a discussion of different ways to compute temperature and perform
thermostatting and barostatting.</p>
<hrclass="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 <aclass="reference internal"href="fix_nh.html"><spanclass="doc">fix npt</span></a> and
<aclass="reference internal"href="fix_nh.html"><spanclass="doc">fix nph</span></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 <aclass="reference internal"href="fix_deform.html"><spanclass="doc">fix deform</span></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 <aclass="reference internal"href="units.html"><spanclass="doc">units</span></a> command).</p>
<divclass="admonition note">
<pclass="first admonition-title">Note</p>
<pclass="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
<p>See the <aclass="reference internal"href="compute_temp.html"><spanclass="doc">compute temp</span></a> and <aclass="reference internal"href="compute_pressure.html"><spanclass="doc">compute pressure</span></a> commands for details. Note that the
IDs of the new computes are the fix-ID + underscore + “temp” or fix_ID
+ underscore + “press”, 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 <aclass="reference internal"href="thermo_style.html"><spanclass="doc">thermo_style</span></a> command) with ID = <em>thermo_temp</em>
and <em>thermo_press</em>. This means you can change the attributes of this
fix’s temperature or pressure via the
<aclass="reference internal"href="compute_modify.html"><spanclass="doc">compute_modify</span></a> command or print this temperature
or pressure during thermodynamic output via the <aclass="reference internal"href="thermo_style.html"><spanclass="doc">thermo_style custom</span></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>
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