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<CENTER><A HREF = "http://lammps.sandia.gov">LAMMPS WWW Site</A> - <A HREF = "Manual.html">LAMMPS Documentation</A> - <A HREF = "Section_commands.html#comm">LAMMPS Commands</A>
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<HR>
<H3>fix press/berendsen command
</H3>
<P><B>Syntax:</B>
</P>
<PRE>fix ID group-ID press/berendsen p-style args keyword value ...
</PRE>
<UL><LI>ID, group-ID are documented in <A HREF = "fix.html">fix</A> command
<LI>press/berendsen = style name of this fix command
<LI>p-style = <I>xyz</I> or <I>xy</I> or <I>yz</I> or <I>xz</I> or <I>aniso</I>
<PRE> <I>xyz</I> args = Pstart Pstop Pdamp
Pstart,Pstop = desired pressure at start/end of run (pressure units)
Pdamp = pressure damping parameter (time units)
<I>xy</I> or <I>yz</I> or <I>xz</I> args = Px0 Px1 Py0 Py1 Pz0 Pz1 Pdamp
Px0,Px1,Py0,Py1,Pz0,Pz1 = desired pressure in x,y,z at
start/end (0/1) of run (pressure units)
Pdamp = pressure damping parameter (time units)
<I>aniso</I> args = Px0 Px1 Py0 Py1 Pz0 Pz1 Pdamp
Px0,Px1,Py0,Py1,Pz0,Pz1 = desired pressure in x,y,z at
start/end (0/1) of run (pressure units)
Pdamp = pressure damping parameter (time units)
</PRE>
<LI>zero or more keyword/value pairs may be appended
<LI>keyword = <I>dilate</I> or <I>modulus</I>
<PRE> <I>dilate</I> value = <I>all</I> or <I>partial</I>
<I>modulus</I> value = bulk modulus of system (pressure units)
</PRE>
</UL>
<P><B>Examples:</B>
</P>
<PRE>fix 1 all press/berendsen xyz 0.0 0.0 1000.0
fix 2 all press/berendsen aniso 0.0 0.0 0.0 0.0 NULL NULL 1000.0 dilate partial
</PRE>
<P><B>Description:</B>
</P>
<P>Reset the pressure of the system by using a Berendsen barostat
<A HREF = "#Berendsen">(Berendsen)</A>, which rescales the system volume and
(optionally) the atoms coordinates withing 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 <I>dilate</I> is specified with a value of <I>partial</I>, 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>
<P>IMPORTANT NOTE: Unlike the <A HREF = "fix_npt.html">fix npt</A> or <A HREF = "fix_nph.html">fix
nph</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 HREF = "fix_nve.html">fix
nve</A> or <A HREF = "fix_nvt.html">fix nvt</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 HREF = "fix_nvt.html">fix nvt</A> or <A HREF = "fix_langevin.html">fix langevin</A> or <A HREF = "fix_temp_berendsen.html">fix
temp/berendsen</A>.
</P>
<P>See <A HREF = "Section_howto.html#4_16">this howto section</A> of the manual for a
discussion of different ways to compute temperature and perform
thermostatting and barostatting.
</P>
<HR>
<P>The pressure can be controlled in one of several styles, as specified
by the <I>p-style</I> argument. In each case, the desired pressure at each
timestep is a ramped value during the run from the starting value to
the end value.
</P>
<P>Style <I>xyz</I> means couple all dimensions together when pressure is
computed (isotropic pressure), and dilate/contract the dimensions
together.
</P>
<P>Styles <I>xy</I> or <I>yz</I> or <I>xz</I> means that the 2 specified dimensions are
coupled together, both for pressure computation and for
dilation/contraction. The 3rd dimension dilates/contracts
independently, using its pressure component as the driving force.
These styles cannot be used for a 2d simulation.
</P>
<P>For style <I>aniso</I>, all dimensions dilate/contract independently using
their individual pressure components as the driving forces.
</P>
<P>For any of the styles except <I>xyz</I>, any of the independent pressure
components (e.g. z in <I>xy</I>, or any dimension in <I>aniso</I>) can have
their target pressures (both start and stop values) specified as NULL.
This means that no pressure control is applied to that dimension so
that the box dimension remains unchanged. For a 2d simulation the z
pressure components must be specified as NULL when using style
<I>aniso</I>.
</P>
<P>For styles <I>xy</I> and <I>yz</I> and <I>xz</I>, the starting and stopping pressures
must be the same for the two coupled dimensions and cannot be
specified as NULL.
</P>
<P>In some cases (e.g. for solids) the pressure (volume) and/or
temperature of the system can oscillate undesirably when a Nose/Hoover
barostat is applied. The optional <I>drag</I> keyword will damp these
oscillations, although it alters the Nose/Hoover equations. A value
of 0.0 (no drag) leaves the Nose/Hoover formalism unchanged. A
non-zero value adds a drag term; the larger the value specified, the
greater the damping effect. Performing a short run and monitoring the
pressure is the best way to determine if the drag term is working.
Typically a value between 0.2 to 2.0 is sufficient to damp
oscillations after a few periods.
</P>
<P>For all pressure styles, the simulation box stays rectangular in
shape. Parinello-Rahman boundary condition for tilted boxes
(triclinic symmetry) are supported by other LAMMPS commands (see <A HREF = "Section_howto.html#4_12">this
section</A> of the manual), but not yet by this
command.
</P>
<P>For all styles, the <I>Pdamp</I> parameter determines the time scale on
which pressure is relaxed. For example, a value of 1000.0 means to
relax the pressure in a timespan of (roughly) 1000 time units (tau or
fmsec or psec - see the <A HREF = "units.html">units</A> command).
</P>
<P>IMPORTANT NOTE: 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's not). The bulk modulus can be set
via the keyword <I>modulus</I>. The <I>Pdamp</I> 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 <I>Pdamp</I>. 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 <I>Pdamp</I> and/or the <I>modulus</I> when using this fix.
</P>
<HR>
<P>This fix computes a temperature and pressure each timestep. To do
this, the fix creates its own computes of style "temp" and "pressure",
as if these commands had been issued:
</P>
<PRE>compute fix-ID_temp group-ID temp
compute fix-ID_press group-ID pressure fix-ID_temp
</PRE>
<P>See the <A HREF = "compute_temp.html">compute temp</A> and <A HREF = "compute_pressure.html">compute
pressure</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 <A HREF = "thermo_style.html">thermo_style</A> command) with ID = <I>thermo_temp</I>
and <I>thermo_press</I>. This means you can change the attributes of this
fix's temperature or pressure via the
<A HREF = "compute_modify.html">compute_modify</A> command or print this temperature
or pressure during thermodynamic output via the <A HREF = "thermo_style.html">thermo_style
custom</A> command using the appropriate compute-ID.
It also means that changing attributes of <I>thermo_temp</I> or
<I>thermo_press</I> will have no effect on this fix.
</P>
<P><B>Restart, fix_modify, output, run start/stop, minimize info:</B>
</P>
<P>No information about this fix is written to <A HREF = "restart.html">binary restart
files</A>.
</P>
<P>The <A HREF = "fix_modify.html">fix_modify</A> <I>temp</I> and <I>press</I> options are
supported by this fix. You can use them to assign a
<A HREF = "compute.html">compute</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 scalar or vector or per-atom quantities are stored by this
fix for access by various <A HREF = "Section_howto.html#4_15">output commands</A>.
</P>
<P>This fix can ramp its target pressure over multiple runs, using the
<I>start</I> and <I>stop</I> keywords of the <A HREF = "run.html">run</A> command. See the
<A HREF = "run.html">run</A> command for details of how to do this.
</P>
<P>This fix is not invoked during <A HREF = "minimize.html">energy minimization</A>.
</P>
<P><B>Restrictions:</B>
</P>
<P>Any dimension being adjusted by this fix must be periodic. A
dimension whose target pressures are specified as NULL can be
non-periodic or periodic.
</P>
<P><B>Related commands:</B>
</P>
<P><A HREF = "fix_nve.html">fix nve</A>, <A HREF = "fix_nph.html">fix nph</A>, <A HREF = "fix_npt.html">fix
npt</A>, <A HREF = "fix_temp_berendsen.html">fix temp/berendsen</A>,
<A HREF = "fix_modify.html">fix_modify</A>
</P>
<P><B>Default:</B>
</P>
<P>The keyword defaults are dilate = all, modulus = 10.0 in units of
pressure for whatever <A HREF = "units.html">units</A> are defined.
</P>
<HR>
<A NAME = "Berendsen"></A>
<P><B>(Berendsen)</B> Berendsen, Postma, van Gunsteren, DiNola, Haak, J Chem
Phys, 81, 3684 (1984).
</P>
</HTML>
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