lammps/doc/fix_nph.txt

"LAMMPS WWW Site"_lws - "LAMMPS Documentation"_ld - "LAMMPS Commands"_lc :c

:link(lws,http://lammps.sandia.gov)
:link(ld,Manual.html)
:link(lc,Section_commands.html#comm)

:line

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 :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 "(Hoover)"_#Hoover, implemented as described in
"(Melchionna)"_#Melchionna.  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 a
thermostatting fis such as "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.

IMPORTANT NOTE: Unlike the "fix
press/berendsen"_fix_press_berendsen.html command which performs
barostatting but NO time integration, this fix performs barostatting
AND time integration.  Thus you should not use any other time
integration fix, such as "fix nve"_fix_nve.html or "fix
nvt"_fix_nvt.html on atoms to which this fix is applied.  Use "fix
npt"_fix_npt.html instead of this fix, if you want to control both
temperature and pressure via Nose/Hoover.

See "this howto section"_Section_howto.html#4_16 of the manual for a
discussion of different ways to compute temperature and perform
thermostatting and barostatting.

: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.

Style {xyz} means couple all dimensions together when pressure is
computed (isotropic pressure), and dilate/contract the 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.
These styles cannot be used for a 2d simulation.

For style {aniso}, all dimensions dilate/contract independently using
their individual pressure components as the 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.  For a 2d simulation the z
pressure components must be specified as NULL when using style
{aniso}.

For styles {xy} and {yz} and {xz}, the starting and stopping pressures
must be the same for the two coupled dimensions and cannot be
specified as NULL.

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 condition for tilted boxes
(triclinic symmetry) are supported by other LAMMPS commands (see "this
section"_Section_howto.html#4_12 of the manual), but not yet by this
command.

For all styles, the {Pdamp} 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 "units"_units.html command).

:line

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 + underscore + "temp" or fix_ID
+ underscore + "press", 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 thermodynamic 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.

[Restart, fix_modify, output, run start/stop, minimize info:]

This fix writes the state of the Nose/Hoover barostat to "binary
restart files"_restart.html.  See the "read_restart"_read_restart.html
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.

The "fix_modify"_fix_modify.html {temp} and {press} options are
supported by this fix.  You can use them to assign a
"compute"_compute.html you have defined to this fix which will be used
in its thermostatting or barostatting procedure.  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.

The "fix_modify"_fix_modify.html {energy} option is supported by this
fix to add the energy change induced by Nose/Hoover barostatting to
the system's potential energy as part of "thermodynamic
output"_thermo_style.html.

The potential energy change due to this fix is stored as a scalar
quantity, which can be accessed by various "output
commands"_Section_howto.html#4_15.  The scalar value calculated by
this fix is "extensive", meaning it scales with the number of atoms in
the simulation.

This fix can ramp its target pressure over multiple runs, using the
{start} and {stop} keywords of the "run"_run.html command.  See the
"run"_run.html command for details of how to do this.

This fix is not invoked during "energy minimization"_minimize.html.

[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.

[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.

:line

:link(Hoover)
[(Hoover)] Hoover, Phys Rev A, 34, 2499 (1986).

:link(Melchionna)
[(Melchionna)] Melchionna, Ciccotti, Holian, Molecular Physics, 78,
533-44 (1993).