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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. 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.

By default, the temperature is also computed for all atoms (used as part of the pressure calculation), regardless of what group is specified. This is because the pressure contains a kinetic energy term which is derived from temperature, and the kinetic energy should be consistent with the virial term computed using all atoms. This can be changed by assigning a different temperature method to the fix via the fix_modify command. 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. 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 is specified in time units and determines how rapidly the pressure is relaxed. For example, a value of 1000.0 means to relax the temperature in a timespan of (roughly) 1000 time units (tau or fmsec or psec - see the units command).

This fix supports the "fix_modify" 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.

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.