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doc/src/fix_wall_reflect_stochastic.rst

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+.. index:: fix wall/reflect/stochastic
+
+fix wall/reflect/stochastic command
+===================================
+
+Syntax
+""""""
+
+
+.. parsed-literal::
+
+   fix ID group-ID wall/reflect/stochastic rstyle seed face args ... keyword value ...
+
+* ID, group-ID are documented in :doc:`fix <fix>` command
+* wall/reflect/stochastic = style name of this fix command
+* rstyle = diffusive or maxwell or ccl
+* seed = random seed for stochasticity (positive integer)
+* one or more face/args pairs may be appended
+* face = *xlo* or *xhi* or *ylo* or *yhi* or *zlo* or *zhi*
+  
+  .. parsed-literal::
+  
+       args = pos temp velx vely velz accomx accomy accomz
+         pos = EDGE or constant
+           EDGE = current lo or hi edge of simulation box
+           constant = number like 0.0 or 30.0 (distance units)
+         temp = wall temperature (temperature units)
+         velx,vely,velz = wall velocity in x,y,z directions (velocity units)
+         accomx,accomy,accomz = accommodation coeffs in x,y,z directions (unitless)
+           not specified for rstyle = diffusive
+           single accom coeff specified for rstyle maxwell
+           all 3 coeffs specified for rstyle cll
+
+* zero or more keyword/value pairs may be appended
+* keyword = *units*
+  
+  .. parsed-literal::
+  
+       *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 zwalls all wall/reflect/stochastic diffusive 23424 zlo EDGE 300 0.1 0.1 0 zhi EDGE 200 0.1 0.1 0
+   fix ywalls all wall/reflect/stochastic maxwell 345533 ylo 5.0 300 0.1 0.0 0.0 0.8 yhi 10.0 300 0.1 0.0 0.0 0.8
+   fix xwalls all wall/reflect/stochastic cercignanilampis 2308 xlo 0.0 300 0.0 0.1 0.9 0.8 0.7 xhi EDGE 300 0.0 0.1 0 0.9 0.8 0.7 units box
+
+Description
+"""""""""""
+
+Bound the simulation with one or more walls which reflect particles
+in the specified group when they attempt to move through them.
+
+Reflection means that if an atom moves outside the wall on a timestep
+(e.g. due to the :doc:`fix nve <fix_nve>` command), then it is put back
+inside the wall with a changed velocity.
+
+This fix models treats the wall as a moving solid boundary with a
+finite temperature, which can exchange energy with particles that
+collide with it.  This is different than the simpler :doc:`fix wall/reflect <fix_wall_reflect>` command which models mirror
+reflection.  For this fix, the post collision velocity of each
+particle is treated stochastically.  The randomness can come from many
+sources: thermal motion of the wall atoms, surface roughness, etc.
+Three stochastic reflection models are currently implemented.
+
+For rstyle *diffusive*\ , particles are reflected diffusively. Their
+velocity distribution corresponds to an equilibrium distribution of
+particles at the wall temperature.  No accommodation coefficients
+are specified.
+
+For rstyle *maxwell*\ , particle reflection is Maxwellian which means
+partially diffusive and partially specular (:ref:`Maxwell <Maxwell>`).  A
+single accommodation coeff is specified which must be between 0.0 and
+1.0 inclusive.  It determines the fraction of the collision which is
+diffusive versus specular.  An accommodation coefficient of 1.0 is fully
+diffusive; a coefficient of 0.0 is fully specular.
+
+For rstyle *cll*\ , particle collisions are computed by the
+Cercignani/Lampis model.  See :ref:`CL <CL>` and :ref:`To <To>` for details.
+Three accommodations coefficient are specified.  Each must be between
+0.0 and 1.0 inclusive.  Two are velocity accommodation coefficients;
+one is a normal kinetic energy accommodation.  The normal coeff is the
+one corresponding to the normal of the wall itself.  For example if
+the wall is *ylo* or *yhi*\ , *accomx* and *accomz* are the tangential
+velocity accommodation coefficients, and *accomy* is the normal
+kinetic energy accommodation coefficient.
+
+The optional *units* keyword determines the distance units used to
+define a wall position.  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.
+
+
+----------
+
+
+Restrictions
+""""""""""""
+
+
+This fix has the same limitations as the :doc:`fix wall/reflect <fix_wall_reflect>` command.  Any dimension (xyz) that
+has a wall must be non-periodic.  It should not be used with rigid
+bodies such as those defined by the :doc:`fix rigid <fix_rigid>`
+command.  The wall velocity must lie on the same plane as the wall
+itself.
+
+This fix is part of the USER-MISC package.  It is only enabled if
+LAMMPS was built with that package.  See the :doc:`Build package <Build_package>` doc page for more info.
+
+Related commands
+""""""""""""""""
+
+:doc:`fix wall/reflect <fix_wall_reflect>`
+
+Default
+"""""""
+
+The default for the units keyword is lattice.
+
+
+----------
+
+
+.. _Maxwell:
+
+
+
+**(Maxwell)** J.C. Maxwell, Philos. Tans. Royal Soc. London, 157: 49-88
+(1867).
+
+.. _CL:
+
+
+
+**(Cercignani)** C. Cercignani and M. Lampis. Trans. Theory
+Stat. Phys. 1, 2, 101 (1971).
+
+.. _To:
+
+
+
+**(To)** Q.D. To, V.H. Vu, G. Lauriat, and
+C. Leonard. J. Math. Phys. 56, 103101 (2015).
+
+
+.. _lws: http://lammps.sandia.gov
+.. _ld: Manual.html
+.. _lc: Commands_all.html