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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 to the args
<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 and Nose/Hoover pressure barostat. P is pressure; T is
temperature. This creates a system trajectory consistent with the
isothermal-isobaric ensemble.
</P>
<P>The desired temperature at each timestep is a ramped value during the
run from <I>Tstart</I> to <I>Tstop</I>. The <A HREF = "run.html">run</A> command documents
how to make the ramping take place across multiple runs. 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>
<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. The <A HREF = "run.html">run</A> command documents how to make the
ramping take place across multiple runs.
</P>
<P>Style <I>xyz</I> means couple all 3 dimensions together when pressure is
computed (isotropic pressure), and dilate/contract the 3 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.
</P>
<P>For style <I>aniso</I>, all 3 dimensions dilate/contract independently
using their individual pressure components as the 3 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.
</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) are not
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<A HREF = "temp"<A HREF = "press"">>>compute
pressure</A> commands for details. Note that the
IDs of the new computes are the fix-ID with </A> or </A>
appended 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 thermodyanmic 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. Alternatively, you
can directly assign a new compute (for calculating temeperature or
pressure) that you have defined to this fix via the
<A HREF = "fix_modify.html">fix_modify</A> command. If you do this, note that the
kinetic energy derived from T should be consistent with the virial
term computed using all atoms. LAMMPS will warn you if you choose to
compute temperature on a subset of atoms.
</P>
<P>This fix makes a contribution to the potential energy of the system
that can be included in thermodynamic output of potential energy using
the <A HREF = "fix_modify.html">fix_modify energy</A> option. The contribution can
also be printed by itself via the keyword <I>f_fix-ID</I> in the
<A HREF = "thermo_style.html">thermo_style custom</A> command.
</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>
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