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doc/html/_sources/fix_nph_sphere.txt
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doc/html/_sources/fix_nph_sphere.txt
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.. index:: fix nph/sphere
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fix nph/sphere command
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======================
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fix nph/sphere/omp command
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==========================
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Syntax
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""""""
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.. parsed-literal::
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fix ID group-ID nph/sphere args keyword value ...
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* ID, group-ID are documented in :doc:`fix <fix>` command
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* nph/sphere = style name of this fix command
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* additional barostat related keyword/value pairs from the :doc:`fix nph <fix_nh>` command can be appended
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Examples
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""""""""
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.. parsed-literal::
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fix 1 all nph/sphere iso 0.0 0.0 1000.0
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fix 2 all nph/sphere x 5.0 5.0 1000.0
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fix 2 all nph/sphere x 5.0 5.0 1000.0 drag 0.2
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fix 2 water nph/sphere aniso 0.0 0.0 1000.0 dilate partial
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Description
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"""""""""""
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Perform constant NPH integration to update position, velocity, and
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angular velocity each timestep for finite-size spherical particles in
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the group using a Nose/Hoover pressure barostat. P is pressure; H is
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enthalpy. This creates a system trajectory consistent with the
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isenthalpic ensemble.
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This fix differs from the :doc:`fix nph <fix_nh>` command, which assumes
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point particles and only updates their position and velocity.
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Additional parameters affecting the barostat are specified by keywords
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and values documented with the :doc:`fix nph <fix_nh>` command. See,
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for example, discussion of the *aniso*\ , and *dilate* keywords.
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The particles in the fix group are the only ones whose velocities and
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positions are updated by the velocity/position update portion of the
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NPH integration.
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Regardless of what particles are in the fix group, a global pressure is
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computed for all particles. Similarly, when the size of the simulation
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box is changed, all particles are re-scaled to new positions, unless the
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keyword *dilate* is specified with a value of *partial*\ , in which case
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only the particles in the fix group are re-scaled. The latter can be
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useful for leaving the coordinates of particles in a solid substrate
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unchanged and controlling the pressure of a surrounding fluid.
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----------
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This fix computes a temperature and pressure each timestep. To do
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this, the fix creates its own computes of style "temp/sphere" and
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"pressure", as if these commands had been issued:
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.. parsed-literal::
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compute fix-ID_temp all temp/sphere
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compute fix-ID_press all pressure fix-ID_temp
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See the :doc:`compute temp/sphere <compute_temp_sphere>` and :doc:`compute pressure <compute_pressure>` commands for details. Note that the
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IDs of the new computes are the fix-ID + underscore + "temp" or fix_ID
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+ underscore + "press", and the group for the new computes is "all"
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since pressure is computed for the entire system.
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Note that these are NOT the computes used by thermodynamic output (see
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the :doc:`thermo_style <thermo_style>` command) with ID = *thermo_temp*
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and *thermo_press*\ . This means you can change the attributes of this
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fix's temperature or pressure via the
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:doc:`compute_modify <compute_modify>` command or print this temperature
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or pressure during thermodynamic output via the :doc:`thermo_style custom <thermo_style>` command using the appropriate compute-ID.
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It also means that changing attributes of *thermo_temp* or
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*thermo_press* will have no effect on this fix.
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----------
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Styles with a *cuda*\ , *gpu*\ , *intel*\ , *kk*\ , *omp*\ , or *opt* suffix are
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functionally the same as the corresponding style without the suffix.
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They have been optimized to run faster, depending on your available
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hardware, as discussed in :doc:`Section_accelerate <Section_accelerate>`
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of the manual. The accelerated styles take the same arguments and
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should produce the same results, except for round-off and precision
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issues.
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These accelerated styles are part of the USER-CUDA, GPU, USER-INTEL,
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KOKKOS, USER-OMP and OPT packages, respectively. They are only
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enabled if LAMMPS was built with those packages. See the :ref:`Making LAMMPS <start_3>` section for more info.
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You can specify the accelerated styles explicitly in your input script
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by including their suffix, or you can use the :ref:`-suffix command-line switch <start_7>` when you invoke LAMMPS, or you can
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use the :doc:`suffix <suffix>` command in your input script.
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See :doc:`Section_accelerate <Section_accelerate>` of the manual for
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more instructions on how to use the accelerated styles effectively.
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Restart, fix_modify, output, run start/stop, minimize info
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""""""""""""""""""""""""""""""""""""""""""""""""""""""""""
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This fix writes the state of the Nose/Hoover barostat to :doc:`binary restart files <restart>`. See the :doc:`read_restart <read_restart>`
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command for info on how to re-specify a fix in an input script that
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reads a restart file, so that the operation of the fix continues in an
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uninterrupted fashion.
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The :doc:`fix_modify <fix_modify>` *temp* and *press* options are
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supported by this fix. You can use them to assign a
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:doc:`compute <compute>` you have defined to this fix which will be used
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in its thermostatting or barostatting procedure. If you do this, note
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that the kinetic energy derived from the compute temperature should be
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consistent with the virial term computed using all atoms for the
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pressure. LAMMPS will warn you if you choose to compute temperature
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on a subset of atoms.
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The :doc:`fix_modify <fix_modify>` *energy* option is supported by this
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fix to add the energy change induced by Nose/Hoover barostatting to
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the system's potential energy as part of :doc:`thermodynamic output <thermo_style>`.
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This fix computes the same global scalar and global vector of
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quantities as does the :doc:`fix nph <fix_nh>` command.
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This fix can ramp its target pressure over multiple runs, using the
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*start* and *stop* keywords of the :doc:`run <run>` command. See the
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:doc:`run <run>` command for details of how to do this.
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This fix is not invoked during :doc:`energy minimization <minimize>`.
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Restrictions
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""""""""""""
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This fix requires that atoms store torque and angular velocity (omega)
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and a radius as defined by the :doc:`atom_style sphere <atom_style>`
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command.
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All particles in the group must be finite-size spheres. They cannot
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be point particles.
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Related commands
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""""""""""""""""
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:doc:`fix nph <fix_nh>`, :doc:`fix nve_sphere <fix_nve_sphere>`, :doc:`fix nvt_sphere <fix_nvt_sphere>`, :doc:`fix npt_sphere <fix_npt_sphere>`,
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:doc:`fix_modify <fix_modify>`
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**Default:** none
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.. _lws: http://lammps.sandia.gov
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.. _ld: Manual.html
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.. _lc: Section_commands.html#comm
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