lammps/doc/fix_nph.txt

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fix nph command :h3

[Syntax:]

fix ID group-ID nph p-style args keyword value ... :pre

ID, group-ID are documented in "fix"_fix.html command :ulb,l
nph = style name of this fix command :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} 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)
  {aniso} 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

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 (0.0 = no drag)
  {dilate} value = {all} or {partial} :pre
:ule

[Examples:]

fix 1 all nph xyz 0.0 0.0 1000.0
fix 2 all nph xz 5.0 5.0 NULL NULL 5.0 5.0 1000.0 drag 1.0
fix 2 all nph aniso 0.0 0.0 0.0 0.0 NULL NULL 1000.0 :pre

[Description:]

Perform constant NPH integration to update positions and velocities
each timestep for atoms in the group using a Nose/Hoover pressure
barostat.  P is pressure.  This creates a system trajectory consistent
with the isobaric ensemble.  Unlike "fix npt"_fix_npt.html,
temperature will not be controlled if no other fix is used.
Temperature can be controlled independently by using ""fix
langevin"_fix_langevin.html or "fix
temp/rescale"_fix_temp_rescale.html.

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.

:line

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 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 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.
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
"units"_units.html command).

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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:

compute fix-ID_temp group-ID temp :pre
compute fix-ID_press group-ID pressure fix-ID_temp :pre

See the "compute temp"_compute_temp.html and "compute
pressure"_compute_pressure.html commands for details.  Note that the
IDs of the new computes are the fix-ID with underscore + "temp" or
"press" appended and the group for the new computes is the same as the
fix group.

Note that these are NOT the computes used by thermodynamic output (see
the "thermo_style"_thermo_style.html command) with ID = {thermo_temp}
and {thermo_press}.  This means you can change the attributes of this
fix's temperature or pressure via the
"compute_modify"_compute_modify.html command or print this temperature
or pressure during thermodyanmic output via the "thermo_style
custom"_thermo_style.html command using the appropriate compute-ID.
It also means that changing attributes of {thermo_temp} or
{thermo_press} 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
"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.

This fix makes a contribution to the potential energy of the system
that can be included in thermodynamic output of potential energy using
the "fix_modify energy"_fix_modify.html option.  The contribution can
also be printed by itself via the keyword {f_fix-ID} in the
"thermo_style custom"_thermo_style.html command.

[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.

You should not use "fix nvt"_fix_nvt.html with this fix.  Instead, use
"fix npt"_fix_npt.html if you want to control both temperature and
pressure via Nose/Hoover.

[Related commands:]

"fix nve"_fix_nve.html, "fix npt"_fix_npt.html,
"fix_modify"_fix_modify.html

[Default:]

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