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fix_wall_piston.txt
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.. index:: fix wall/piston
fix wall/piston command
=======================
Syntax
""""""
.. parsed-literal::
fix ID group-ID wall/piston face ... keyword value ...
* ID, group-ID are documented in :doc:`fix <fix>` command
* wall/piston = style name of this fix command
* face = *zlo*
* zero or more keyword/value pairs may be appended
* keyword = *pos* or *vel* or *ramp* or *units*
.. parsed-literal::
*pos* args = z
z = z coordinate at which the piston begins (distance units)
*vel* args = vz
vz = final velocity of the piston (velocity units)
*ramp* = use a linear velocity ramp from 0 to vz
*temp* args = target damp seed extent
target = target velocity for region immediately ahead of the piston
damp = damping paramter (time units)
seed = random number seed for langevin kicks
extent = extent of thermostated region (distance units)
*units* value = *lattice* or *box*
*lattice* = the wall position is defined in lattice units
*box* = the wall position is defined in simulation box units
Examples
""""""""
.. parsed-literal::
fix xwalls all wall/piston zlo
fix walls all wall/piston zlo pos 1.0 vel 10.0 units box
fix top all wall/piston zlo vel 10.0 ramp
Description
"""""""""""
Bound the simulation with a moving wall which reflect particles in the
specified group and drive the system with an effective infinite-mass
piston capable of driving shock waves.
A momentum mirror technique is used, which means that if an atom (or
the wall) moves such that an atom is outside the wall on a timestep by
a distance delta (e.g. due to :doc:`fix nve <fix_nve>`), then it is put
back inside the face by the same delta, and the velocity relative to
the moving wall is flipped in z. For instance, a stationary particle
hit with a piston wall with velocity vz, will end the timestep with a
velocity of 2*vz.
Currently the *face* keyword can only be *zlo*. This creates a piston
moving in the positive z direction. Particles with z coordinate less
than the wall position are reflected to a z coordinate greater than
the wall position. If the piston velocity is vpz and the particle
velocity before reflection is vzi, the particle velocity after
reflection is -vzi + 2*vpz.
The initial position of the wall can be specified by the *pos* keyword.
The final velocity of the wall can be specified by the *vel* keyword
The *ramp* keyword will cause the wall/piston to adjust the velocity
linearly from zero velocity to *vel* over the course of the run. If
the *ramp* keyword is omitted then the wall/piston moves at a constant
velocity defined by *vel*.
The *temp* keyword will cause the region immediately in front of the
wall/piston to be thermostated with a Langevin thermostat. This
region moves with the piston. The damping and kicking are measured in
the reference frame of the piston. So, a temperature of zero would
mean all particles were moving at exactly the speed of the
wall/piston.
The *units* keyword determines the meaning of the distance units used
to define a wall position, but only when a numeric constant is used.
A *box* value selects standard distance units as defined by the
:doc:`units <units>` command, e.g. Angstroms for units = real or metal.
A *lattice* value means the distance units are in lattice spacings.
The :doc:`lattice <lattice>` command must have been previously used to
define the lattice spacings.
----------
Restart, fix_modify, output, run start/stop, minimize info
""""""""""""""""""""""""""""""""""""""""""""""""""""""""""
No information about this fix is written to :doc:`binary restart files <restart>`. None of the :doc:`fix_modify <fix_modify>` options
are relevant to this fix. No global or per-atom quantities are stored
by this fix for access by various :ref:`output commands <howoto_15>`. No parameter of this fix can
be used with the *start/stop* keywords of the :doc:`run <run>` command.
This fix is not invoked during :doc:`energy minimization <minimize>`.
Restrictions
""""""""""""
This fix style is part of the SHOCK package. It is only enabled if
LAMMPS was built with that package. See the :ref:`Making LAMMPS <start_3>` section for more info.
The face that has the wall/piston must be boundary type 's'
(shrink-wrapped). The opposing face can be
any boundary type other than periodic.
A wall/piston should not be used with rigid bodies such as those
defined by a "fix rigid" command. This is because the wall/piston
displaces atoms directly rather than exerting a force on them.
Related commands
""""""""""""""""
:doc:`fix wall/reflect <fix_wall>` command, :doc:`fix append/atoms <fix_append_atoms>` command
Default
"""""""
The keyword defaults are pos = 0, vel = 0, units = lattice.
.. _lws: http://lammps.sandia.gov
.. _ld: Manual.html
.. _lc: Section_commands.html#comm
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