lammps/doc/fix_npt.txt

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fix npt command :h3

[Syntax:]

fix ID group-ID npt Tstart Tstop Tdamp p-style args keyword value ... :pre

ID, group-ID are documented in "fix"_fix.html command :ulb,l
npt = style name of this fix command :l
Tstart,Tstop = desired temperature at start/end of run :l
Tdamp = temperature damping parameter (time units) :l
p-style = {xyz} or {xy} or {yz} or {xz} or {aniso} :l
  {xyz} args = Pstart Pstop Pdamp
    Pstart,Pstop = desired pressure at start/end of run (pressure units)
    Pdamp = pressure damping parameter (time units)
  {xy} or {yz} or {xz} or {aniso} 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

zero or more keyword/value pairs may be appended to the args :l
keyword = {drag} or {dilate} :l
  {drag} value = drag factor added to barostat/thermostat (0.0 = no drag)
  {dilate} value = {all} or {partial} :pre
:ule

[Examples:]

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

[Description:]

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.

The desired temperature at each timestep is a ramped value during the
run from {Tstart} to {Tstop}.  The "run"_run.html command documents
how to make the ramping take place across multiple runs.  The {Tdamp}
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 "units"_units.html command).

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.

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 {dilate} is specified with a value of {partial}, 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.

This fix computes a temperature each timestep, to contribute to the
pressure.  The fix creates its own method for computing T, as if it
had been defined by the command:

temperature fix-ID all full :pre

See the "temperature"_temperature.html command for details.  Note that
this is NOT the temperature with ID = {default}.  This means you can
change the attributes of this fix's temperature (e.g. its
degrees-of-freedom) via the "temp_modify"_temp_modify.html command or
print the temperature with thermodyanmic output via the "thermo_style
custom"_thermo_style.html command using the appropriate temp-ID =
fix-ID.  It also means that changing attributes of the default
temperature will have no effect on this fix.  Alternatively, you can
directly assign a new temperature to the fix via the
"fix_modify"_fix_modify.html 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.

The pressure can be controlled in one of several styles, as specified
by the {p-style} 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 "run"_run.html command documents how to make the
ramping take place across multiple runs.

Style {xyz} means couple all 3 dimensions together when pressure is
computed (isotropic pressure), and dilate/contract the 3 dimensions
together.

Styles {xy} or {yz} or {xz} 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.

For style {aniso}, all 3 dimensions dilate/contract independently
using their individual pressure components as the 3 driving forces.

For any of the styles except {xyz}, any of the independent pressure
components (e.g. z in {xy}, or any dimension in {aniso}) 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.

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 {drag} 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.

For all pressure styles, the simulation box stays rectangular in
shape.  Parinello-Rahman boundary conditions (tilted box) are not
implemented in LAMMPS.

For all styles, the {Pdamp} parameter operates like the {Tdamp}
parameter, determining the time scale on which pressure is relaxed.

This fix supports the "fix_modify"_fix_modify.html options for
{thermo} and {energy}.  The former will print the contribution the fix
makes to the energy of the system when thermodynamics is printed.  The
latter will add this contribution to the total potential energy
(PotEng) so that energy conservation can be monitored.

[Restrictions:]

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.

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.

[Related commands:]

"fix nve"_fix_nve.html, "fix nvt"_fix_nvt.html, "fix nph"_fix_nph.html,
"fix_modify"_fix_modify.html

[Default:]

The keyword defaults are drag = 0.0 and dilate = all.