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fix npt command

Syntax:

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

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 

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 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 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 

See the temperature 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 command or print the temperature with thermodyanmic output via the thermo_style custom 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 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 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 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 nvt, fix nph, fix_modify

Default:

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