---------------------------------- LICENSE ------------------------------------

Copyright 2012-2017 Hanno Dietrich. This software is distributed under the GNU
General Public License. It is an open-source code, distributed freely under the
terms of the GNU Public License (GPL).

-------------------------------- DISCLAIMER -----------------------------------

This software is supplied "AS IS" without any warranties and support. We do
neither assume any responsibility or liability for the use of the software, nor
convey any license or title under any patent, copyright, or mask work right to
the product.

We reserve the right to make changes in the software without notification.
We also make no representation or warranty that such application will be
suitable for the specified use without further testing or modification.

------------------------------- DOCUMENTATION ---------------------------------

In order to add this feature to LAMMPS simply copy the files in the src/
directory to the src/ directory of your LAMMPS distribution and re-compile
LAMMPS.

The format is simalar to that of the fix/wall command:

fix ID group-ID style face args ... keyword value ...

- ID, group-ID are documented in fix command
- style = wall/pressure/lj93 or wall/pressure/lj126 or wall/pressure/lj1043
  or wall/pressure/harmonic
- one or more face/arg pairs may be appended
- face = xlo or xhi or ylo or yhi or zlo or zhi

args = coord epsilon sigma cutoff pressure density damping
  coord = position of wall
  epsilon = strength factor for wall-particle interaction
    (energy or energy/distance^2 units)
    epsilon can be a variable (see below)
  sigma = size factor for wall-particle interaction
    (distance units)
    sigma can be a variable (see below)
  cutoff = distance from wall at which wall-particle interaction is cut off
    (distance units)
  density = area density 
    (mass units per (length unit)^2)
  pressure = pressure applied to the wall
    (pressure units)
    pressure can be a variable (see below)
  damping = damping factor applied to the wall's velocity to prevent "hopping"
    must be in the range 0 to 1 with reasonable values close to 1

example:
fix piston all wall/pressure/harmonic zhi 71.0 0.20 0.0 5 1.0 1 0.995