LAMMPS WWW Site - LAMMPS Documentation - LAMMPS Commands

fix nph command

Syntax:

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

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 

Description:

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

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.


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.

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 command).


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 
compute fix-ID_press group-ID pressure fix-ID_temp 

See the compute temp and compute pressure 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 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 command or print this temperature or pressure during thermodyanmic output via the thermo_style custom 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. See the read_restart 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 temp and press options are supported by this fix. You can use them to assign a compute 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 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.

The potential energy change due to this fix is stored as a scalar quantity, which can be accessed by various output commands. 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 command. See the run command for details of how to do this.

This fix is not invoked during energy minimization.

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 with this fix. Instead, use fix npt if you want to control both temperature and pressure via Nose/Hoover.

Related commands:

fix nve, fix npt, fix_modify

Default:

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


(Hoover) Hoover, Phys Rev A, 34, 2499 (1986).

(Melchionna) Melchionna, Ciccotti, Holian, Molecular Physics, 78, 533-44 (1993).