\(\renewcommand{\AA}{\text{Å}}\)

# fix nph/body command

## Syntax

    fix ID group-ID nph/body args keyword value ...

- ID, group-ID are documented in fix command

- nph/body = style name of this fix command

- additional barostat related keyword/value pairs from the fix nph command can be appended

## Examples

    fix 1 all nph/body iso 0.0 0.0 1000.0
    fix 2 all nph/body x 5.0 5.0 1000.0
    fix 2 all nph/body x 5.0 5.0 1000.0 drag 0.2
    fix 2 water nph/body aniso 0.0 0.0 1000.0 dilate partial

## Description

Perform constant NPH integration to update position, velocity, orientation, and angular velocity each timestep for body particles in the group using a Nose/Hoover pressure barostat. P is pressure; H is enthalpy. This creates a system trajectory consistent with the isenthalpic ensemble.

This fix differs from the fix nph command, which assumes point particles and only updates their position and velocity.

Additional parameters affecting the barostat are specified by keywords and values documented with the fix nph command. See, for example, discussion of the aniso, and dilate keywords.

The particles in the fix group are the only ones whose velocities and positions are updated by the velocity/position update portion of the NPH integration.

Regardless of what particles are in the fix group, a global pressure is computed for all particles. Similarly, when the size of the simulation box is changed, all particles are re-scaled to new positions, unless the keyword dilate is specified with a value of partial, in which case only the particles in the fix group are re-scaled. The latter can be useful for leaving the coordinates of particles in a solid substrate unchanged and controlling the pressure of a surrounding fluid.

This fix computes a temperature and pressure each timestep. To do this, the fix creates its own computes of style “temp/body” and “pressure”, as if these commands had been issued:

    compute fix-ID_temp all temp/body
    compute fix-ID_press all pressure fix-ID_temp

See the compute temp/body 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 “all” since pressure is computed for the entire system.

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

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 cumulative energy change in the system imposed by this fix is included in the thermodynamic output keywords ecouple and econserve. See the thermo_style doc page for details.

This fix computes the same global scalar and global vector of quantities as does the fix nph command.

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

This fix is part of the BODY package. It is only enabled if LAMMPS was built with that package. See the Build package page for more info.

This fix requires that atoms store torque and angular momentum and a quaternion as defined by the atom_style body command.

## Related commands

fix nph, fix nve_body, fix nvt_body, fix npt_body, fix_modify

## Default

none