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### fix press/berendsen command

Syntax:

    fix ID group-ID press/berendsen p-style args keyword value ...

- ID, group-ID are documented in fix command
- press/berendsen = style name of this fix command
- 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 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)

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

        dilate value = all or partial
        modulus value = bulk modulus of system (pressure units)

Examples:

    fix 1 all press/berendsen xyz 0.0 0.0 1000.0
    fix 2 all press/berendsen aniso 0.0 0.0 0.0 0.0 NULL NULL 1000.0 dilate partial

Description:

Reset the pressure of the system by using a Berendsen barostat (Berendsen), which rescales the system volume and (optionally) the atoms coordinates withing the simulation box every timestep.

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 npt or fix nph commands which perform Nose/Hoover barostatting AND time integration, this fix does NOT perform time integration. It only modifies the box size and atom coordinates to effect barostatting. Thus you must use a separate time integration fix, like fix nve or fix nvt to actually update the positions and velocities of atoms. This fix can be used in conjunction with thermostatting fixes to control the temperature, such as fix nvt or fix langevin or fix temp/berendsen.

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 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 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 command).

IMPORTANT NOTE: The relaxation time is actually also a function of the bulk modulus of the system (inverse of isothermal compressibility). The bulk modulus has units of pressure and is the amount of pressure that would need to be applied (isotropically) to reduce the volume of the system by a factor of 2 (assuming the bulk modulus was a constant, independent of density, which it's not). The bulk modulus can be set via the keyword modulus. The Pdamp parameter is effectively multiplied by the bulk modulus, so if the pressure is relaxing faster than expected or desired, increasing the bulk modulus has the same effect as increasing Pdamp. The converse is also true. LAMMPS does not attempt to guess a correct value of the bulk modulus; it just uses 10.0 as a default value which gives reasonable relaxation for a Lennard-Jones liquid, but will be way off for other materials and way too small for solids. Thus you should experiment to find appropriate values of Pdamp and/or the modulus when using this fix.

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.

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

No information about this fix is written to binary restart files.

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 temperature and pressure calculations. 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.

No global scalar or vector or per-atom quantities are stored by this fix for access by various output commands.

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.

Related commands:

fix nve, fix nph, fix npt, fix temp/berendsen, fix_modify

Default:

The keyword defaults are dilate = all, modulus = 10.0 in units of pressure for whatever units are defined.

(Berendsen) Berendsen, Postma, van Gunsteren, DiNola, Haak, J Chem Phys, 81, 3684 (1984).