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### fix npt command

Syntax:

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

- ID, group-ID are documented in fix command
- npt = style name of this fix command
- Tstart,Tstop = desired temperature at start/end of run (temperature units)
- Tdamp = temperature damping parameter (time units)
- p-style = xyz or xy or yz or xz or aniso

        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 or aniso args = Px_start Px_stop Py_start Py_stop Pz_start Pz_stop Pdamp
          Px_start,Px_stop,... = desired pressure in x,y,z at start/end of run (pressure units)
          Pdamp = pressure damping parameter (time units)

- zero or more keyword/value pairs may be appended
- keyword = drag or dilate

        drag value = drag factor added to barostat/thermostat (0.0 = no drag)
        dilate value = all or partial

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 (Hoover1) and Nose/Hoover pressure barostat (Hoover2), implemented as described in (Melchionna). P is pressure; T is temperature. This creates a system trajectory consistent with the isothermal-isobaric ensemble.

The thermostat is applied to only the translational degrees of freedom for the particles. The translational degrees of freedom can also have a bias velocity removed from them before thermostatting takes place; see the description below.

The desired temperature at each timestep is a ramped value during the run from Tstart to Tstop. 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.

IMPORTANT NOTE: Unlike the fix temp/berendsen command which performs thermostatting but NO time integration, this fix performs thermostatting/barostatting AND time integration. Thus you should not use any other time integration fix, such as fix nve on atoms to which this fix is applied. Likewise, this fix should not normally be used on atoms that also have their temperature controlled by another fix - e.g. by fix langevin or fix temp/rescale commands.

See this howto section of the manual for a discussion of different ways to compute temperature and perform thermostatting and barostatting.

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

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 thermodynamic 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.

Like other fixes that perform thermostatting, this fix can be used with compute commands that calculate a temperature after removing a "bias" from the atom velocities. E.g. removing the center-of-mass velocity from a group of atoms or only calculating temperature on the x-component of velocity or only calculating temperature for atoms in a geometric region. This is not done by default, but only if the fix_modify command is used to assign a temperature compute to this fix that includes such a bias term. See the doc pages for individual compute commands to determine which ones include a bias. In this case, the thermostat works in the following manner: the current temperature is calculated taking the bias into account, bias is removed from each atom, thermostatting is performed on the remaining thermal degrees of freedom, and the bias is added back in.

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

This fix writes the state of the Nose/Hoover thermostat and 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, as described above. 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.

IMPORTANT NOTE: If both the temp and press keywords are used in a single thermo_modify command (or in two separate commands), then the order in which the keywords are specified is important. Note that a pressure compute defines its own temperature compute as an argument when it is specified. The temp keyword will override this (for the pressure compute being used by fix npt), but only if the temp keyword comes after the press keyword. If the temp keyword comes before the press keyword, then the new pressure compute specified by the press keyword will be unaffected by the temp setting.

The fix_modify energy option is supported by this fix to add the energy change induced by Nose/Hoover thermostatting and barostatting to the system's potential energy as part of thermodynamic output.

The cummulative 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".

This fix can ramp its target temperature and 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.

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.

(Hoover1) Hoover, Phys Rev A, 31, 1695 (1985).

(Hoover2) Hoover, Phys Rev A, 34, 2499 (1986).

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