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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 npt command
</H3>
<P><B>Syntax:</B>
</P>
<PRE>fix ID group-ID npt Tstart Tstop Tdamp p-style args keyword value ...
</PRE>
<UL><LI>ID, group-ID are documented in <A HREF = "fix.html">fix</A> command
<LI>npt = style name of this fix command
<LI>Tstart,Tstop = desired temperature at start/end of run
<LI>Tdamp = temperature damping parameter (time units)
<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> or <I>aniso</I> args = Px_start Px_stop Py_start Py_stop Pz_start Pz_stop Pdamp
Px_start,Px_stop,... = desired pressure in x,y,z at start/end of run (pressure units)
Pdamp = pressure damping parameter (time units)
</PRE>
<LI>zero or more keyword/value pairs may be appended
<LI>keyword = <I>drag</I> or <I>dilate</I>
<PRE> <I>drag</I> value = drag factor added to barostat/thermostat (0.0 = no drag)
<I>dilate</I> value = <I>all</I> or <I>partial</I>
</PRE>
</UL>
<P><B>Examples:</B>
</P>
<PRE>fix 1 all npt 300.0 300.0 100.0 xyz 0.0 0.0 1000.0
fix 2 all npt 300.0 300.0 100.0 xz 5.0 5.0 NULL NULL 5.0 5.0 1000.0
fix 2 all npt 300.0 300.0 100.0 xz 5.0 5.0 NULL NULL 5.0 5.0 1000.0 drag 0.2
fix 2 water npt 300.0 300.0 100.0 aniso 0.0 0.0 0.0 0.0 NULL NULL 1000.0 dilate partial
</PRE>
<P><B>Description:</B>
</P>
<P>Perform constant NPT integration to update positions and velocities
each timestep for atoms in the group using a Nose/Hoover temperature
thermostat <A HREF = "#Hoover1">(Hoover1)</A> and Nose/Hoover pressure barostat
<A HREF = "#Hoover2">(Hoover2)</A>, implemented as described in
<A HREF = "#Melchionna">(Melchionna)</A>. P is pressure; T is temperature. This
creates a system trajectory consistent with the isothermal-isobaric
ensemble.
</P>
<P>The thermostat is applied to only the translational degrees of freedom
for the particles. The translational degrees of freedom can also have
a bias velocity removed from them before thermostatting takes place;
see the description below.
</P>
<P>The desired temperature at each timestep is a ramped value during the
run from <I>Tstart</I> to <I>Tstop</I>. The <I>Tdamp</I> parameter is specified in
time units and determines how rapidly the temperature is relaxed. For
example, a value of 100.0 means to relax the temperature in a timespan
of (roughly) 100 time units (tau or fmsec or psec - see the
<A HREF = "units.html">units</A> command).
</P>
<P>The atoms in the fix group are the only ones whose velocities and
positions are updated by the velocity/position update portion of the
NPT integration.
</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_temp_berendsen.html">fix
temp/berendsen</A> command which performs
thermostatting but NO time integration, this fix performs
thermostatting/barostatting AND time integration. Thus you should not
use any other time integration fix, such as <A HREF = "fix_nve.html">fix nve</A> on
atoms to which this fix is applied. Likewise, this fix should not
normally be used on atoms that also have their temperature controlled
by another fix - e.g. by <A HREF = "fix_nvt.html">fix langevin</A> or <A HREF = "fix_temp_rescale.html">fix
temp/rescale</A> commands.
</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>In some cases (e.g. for solids) the pressure (volume) and/or
temperature of the system can oscillate undesirably when a Nose/Hoover
barostat and thermostat 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 and temperature 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 conditions (tilted box) for this fix
are not yet implemented in LAMMPS.
</P>
<P>For all styles, the <I>Pdamp</I> parameter operates like the <I>Tdamp</I>
parameter, determining 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>
<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>Like other fixes that perform thermostatting, this fix can be used
with <A HREF = "compute.html">compute commands</A> that calculate a temperature
after removing a "bias" from the atom velocities. E.g. removing the
center-of-mass velocity from a group of atoms or only calculating
temperature on the x-component of velocity or only calculating
temperature for atoms in a geometric region. This is not done by
default, but only if the <A HREF = "fix_modify.html">fix_modify</A> command is used
to assign a temperature compute to this fix that includes such a bias
term. See the doc pages for individual <A HREF = "compute.html">compute
commands</A> to determine which ones include a bias. In
this case, the thermostat works in the following manner: the current
temperature is calculated taking the bias into account, bias is
removed from each atom, thermostatting is performed on the remaining
thermal degrees of freedom, and the bias is added back in.
</P>
<P><B>Restart, fix_modify, output, run start/stop, minimize info:</B>
</P>
<P>This fix writes the state of the Nose/Hoover thermostat and barostat
to <A HREF = "restart.html">binary restart files</A>. See the
<A HREF = "read_restart.html">read_restart</A> command for info on how to re-specify
a fix in an input script that reads a restart file, so that the
operation of the fix continues in an uninterrupted fashion.
</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 thermostatting or barostatting procedure, as described above.
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>IMPORTANT NOTE: If both the <I>temp</I> and <I>press</I> keywords are used in a
single thermo_modify command (or in two separate commands), then the
order in which the keywords are specified is important. Note that a
<A HREF = "compute_pressure.html">pressure compute</A> defines its own temperature
compute as an argument when it is specified. The <I>temp</I> keyword will
override this (for the pressure compute being used by fix npt), but
only if the <I>temp</I> keyword comes after the <I>press</I> keyword. If the
<I>temp</I> keyword comes before the <I>press</I> keyword, then the new pressure
compute specified by the <I>press</I> keyword will be unaffected by the
<I>temp</I> setting.
</P>
<P>The <A HREF = "fix_modify.html">fix_modify</A> <I>energy</I> option is supported by this
fix to add the energy change induced by Nose/Hoover thermostatting and
barostatting to the system's potential energy as part of
<A HREF = "thermo_style.html">thermodynamic output</A>.
</P>
<P>The potential energy change due to this fix is stored as a scalar
quantity, which can be accessed by various <A HREF = "Section_howto.html#4_15">output
commands</A>. The scalar value calculated by
this fix is "extensive", meaning it scales with the number of atoms in
the simulation.
</P>
<P>This fix can ramp its target temperature and 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>The final Tstop cannot be 0.0 since it would make the target T = 0.0
at some timestep during the simulation which is not allowed in
the Nose/Hoover formulation.
</P>
<P><B>Related commands:</B>
</P>
<P><A HREF = "fix_nve.html">fix nve</A>, <A HREF = "fix_nvt.html">fix nvt</A>, <A HREF = "fix_nph.html">fix nph</A>,
<A HREF = "fix_modify.html">fix_modify</A>
</P>
<P><B>Default:</B>
</P>
<P>The keyword defaults are drag = 0.0 and dilate = all.
</P>
<HR>
<A NAME = "Hoover1"></A>
<P><B>(Hoover1)</B> Hoover, Phys Rev A, 31, 1695 (1985).
</P>
<A NAME = "Hoover2"></A>
<P><B>(Hoover2)</B> Hoover, Phys Rev A, 34, 2499 (1986).
</P>
<A NAME = "Melchionna"></A>
<P><B>(Melchionna)</B> Melchionna, Ciccotti, Holian, Molecular Physics, 78,
533-44 (1993).
</P>
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