2016-05-10 01:33:12 +08:00
<!DOCTYPE html>
<!-- [if IE 8]><html class="no - js lt - ie9" lang="en" > <![endif] -->
<!-- [if gt IE 8]><! --> < html class = "no-js" lang = "en" > <!-- <![endif] -->
< head >
< meta charset = "utf-8" >
< meta name = "viewport" content = "width=device-width, initial-scale=1.0" >
< title > fix wall/piston command — LAMMPS documentation< / title >
< link rel = "stylesheet" href = "_static/css/theme.css" type = "text/css" / >
< link rel = "stylesheet" href = "_static/sphinxcontrib-images/LightBox2/lightbox2/css/lightbox.css" type = "text/css" / >
< link rel = "top" title = "LAMMPS documentation" href = "index.html" / >
< script src = "_static/js/modernizr.min.js" > < / script >
< / head >
< body class = "wy-body-for-nav" role = "document" >
< div class = "wy-grid-for-nav" >
< nav data-toggle = "wy-nav-shift" class = "wy-nav-side" >
< div class = "wy-side-nav-search" >
< a href = "Manual.html" class = "icon icon-home" > LAMMPS
< / a >
< div role = "search" >
< form id = "rtd-search-form" class = "wy-form" action = "search.html" method = "get" >
< input type = "text" name = "q" placeholder = "Search docs" / >
< input type = "hidden" name = "check_keywords" value = "yes" / >
< input type = "hidden" name = "area" value = "default" / >
< / form >
< / div >
< / div >
< div class = "wy-menu wy-menu-vertical" data-spy = "affix" role = "navigation" aria-label = "main navigation" >
< ul >
< li class = "toctree-l1" > < a class = "reference internal" href = "Section_intro.html" > 1. Introduction< / a > < / li >
< li class = "toctree-l1" > < a class = "reference internal" href = "Section_start.html" > 2. Getting Started< / a > < / li >
< li class = "toctree-l1" > < a class = "reference internal" href = "Section_commands.html" > 3. Commands< / a > < / li >
< li class = "toctree-l1" > < a class = "reference internal" href = "Section_packages.html" > 4. Packages< / a > < / li >
< li class = "toctree-l1" > < a class = "reference internal" href = "Section_accelerate.html" > 5. Accelerating LAMMPS performance< / a > < / li >
< li class = "toctree-l1" > < a class = "reference internal" href = "Section_howto.html" > 6. How-to discussions< / a > < / li >
< li class = "toctree-l1" > < a class = "reference internal" href = "Section_example.html" > 7. Example problems< / a > < / li >
< li class = "toctree-l1" > < a class = "reference internal" href = "Section_perf.html" > 8. Performance & scalability< / a > < / li >
< li class = "toctree-l1" > < a class = "reference internal" href = "Section_tools.html" > 9. Additional tools< / a > < / li >
< li class = "toctree-l1" > < a class = "reference internal" href = "Section_modify.html" > 10. Modifying & extending LAMMPS< / a > < / li >
< li class = "toctree-l1" > < a class = "reference internal" href = "Section_python.html" > 11. Python interface to LAMMPS< / a > < / li >
< li class = "toctree-l1" > < a class = "reference internal" href = "Section_errors.html" > 12. Errors< / a > < / li >
< li class = "toctree-l1" > < a class = "reference internal" href = "Section_history.html" > 13. Future and history< / a > < / li >
< / ul >
< / div >
< / nav >
< section data-toggle = "wy-nav-shift" class = "wy-nav-content-wrap" >
< nav class = "wy-nav-top" role = "navigation" aria-label = "top navigation" >
< i data-toggle = "wy-nav-top" class = "fa fa-bars" > < / i >
< a href = "Manual.html" > LAMMPS< / a >
< / nav >
< div class = "wy-nav-content" >
< div class = "rst-content" >
< div role = "navigation" aria-label = "breadcrumbs navigation" >
< ul class = "wy-breadcrumbs" >
< li > < a href = "Manual.html" > Docs< / a > » < / li >
< li > fix wall/piston command< / li >
< li class = "wy-breadcrumbs-aside" >
< a href = "http://lammps.sandia.gov" > Website< / a >
< a href = "Section_commands.html#comm" > Commands< / a >
< / li >
< / ul >
< hr / >
< / div >
< div role = "main" class = "document" itemscope = "itemscope" itemtype = "http://schema.org/Article" >
< div itemprop = "articleBody" >
< div class = "section" id = "fix-wall-piston-command" >
< span id = "index-0" > < / span > < h1 > fix wall/piston command< / h1 >
< div class = "section" id = "syntax" >
< h2 > Syntax< / h2 >
< div class = "highlight-default" > < div class = "highlight" > < pre > < span > < / span > < span class = "n" > fix< / span > < span class = "n" > ID< / span > < span class = "n" > group< / span > < span class = "o" > -< / span > < span class = "n" > ID< / span > < span class = "n" > wall< / span > < span class = "o" > /< / span > < span class = "n" > piston< / span > < span class = "n" > face< / span > < span class = "o" > ...< / span > < span class = "n" > keyword< / span > < span class = "n" > value< / span > < span class = "o" > ...< / span >
< / pre > < / div >
< / div >
< ul class = "simple" >
< li > ID, group-ID are documented in < a class = "reference internal" href = "fix.html" > < span class = "doc" > fix< / span > < / a > command< / li >
< li > wall/piston = style name of this fix command< / li >
< li > face = < em > zlo< / em > < / li >
< li > zero or more keyword/value pairs may be appended< / li >
< li > keyword = < em > pos< / em > or < em > vel< / em > or < em > ramp< / em > or < em > units< / em > < / li >
< / ul >
< pre class = "literal-block" >
< em > pos< / em > args = z
z = z coordinate at which the piston begins (distance units)
< em > vel< / em > args = vz
vz = final velocity of the piston (velocity units)
< em > ramp< / em > = use a linear velocity ramp from 0 to vz
< em > temp< / em > 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)
< em > units< / em > value = < em > lattice< / em > or < em > box< / em >
< em > lattice< / em > = the wall position is defined in lattice units
< em > box< / em > = the wall position is defined in simulation box units
< / pre >
< / div >
< div class = "section" id = "examples" >
< h2 > Examples< / h2 >
< div class = "highlight-default" > < div class = "highlight" > < pre > < span > < / span > < span class = "n" > fix< / span > < span class = "n" > xwalls< / span > < span class = "nb" > all< / span > < span class = "n" > wall< / span > < span class = "o" > /< / span > < span class = "n" > piston< / span > < span class = "n" > zlo< / span >
< span class = "n" > fix< / span > < span class = "n" > walls< / span > < span class = "nb" > all< / span > < span class = "n" > wall< / span > < span class = "o" > /< / span > < span class = "n" > piston< / span > < span class = "n" > zlo< / span > < span class = "n" > pos< / span > < span class = "mf" > 1.0< / span > < span class = "n" > vel< / span > < span class = "mf" > 10.0< / span > < span class = "n" > units< / span > < span class = "n" > box< / span >
< span class = "n" > fix< / span > < span class = "n" > top< / span > < span class = "nb" > all< / span > < span class = "n" > wall< / span > < span class = "o" > /< / span > < span class = "n" > piston< / span > < span class = "n" > zlo< / span > < span class = "n" > vel< / span > < span class = "mf" > 10.0< / span > < span class = "n" > ramp< / span >
< / pre > < / div >
< / div >
< / div >
< div class = "section" id = "description" >
< h2 > Description< / h2 >
< p > 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.< / p >
< p > 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
2016-05-10 03:49:32 +08:00
a distance delta (e.g. due to < a class = "reference internal" href = "fix_nve.html" > < span class = "doc" > fix nve< / span > < / a > ), then it is put
2016-05-10 01:33:12 +08:00
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.< / p >
< p > Currently the < em > face< / em > keyword can only be < em > zlo< / em > . 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.< / p >
< p > The initial position of the wall can be specified by the < em > pos< / em > keyword.< / p >
< p > The final velocity of the wall can be specified by the < em > vel< / em > keyword< / p >
< p > The < em > ramp< / em > keyword will cause the wall/piston to adjust the velocity
linearly from zero velocity to < em > vel< / em > over the course of the run. If
the < em > ramp< / em > keyword is omitted then the wall/piston moves at a constant
velocity defined by < em > vel< / em > .< / p >
< p > The < em > temp< / em > 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.< / p >
< p > The < em > units< / em > keyword determines the meaning of the distance units used
to define a wall position, but only when a numeric constant is used.< / p >
< p > A < em > box< / em > value selects standard distance units as defined by the
2016-05-10 05:12:06 +08:00
< a class = "reference internal" href = "units.html" > < span class = "doc" > units< / span > < / a > command, e.g. Angstroms for units = real or metal.
2016-05-10 01:33:12 +08:00
A < em > lattice< / em > value means the distance units are in lattice spacings.
The < a class = "reference internal" href = "lattice.html" > < span class = "doc" > lattice< / span > < / a > command must have been previously used to
define the lattice spacings.< / p >
< / div >
< hr class = "docutils" / >
< div class = "section" id = "restart-fix-modify-output-run-start-stop-minimize-info" >
< h2 > Restart, fix_modify, output, run start/stop, minimize info< / h2 >
< p > No information about this fix is written to < a class = "reference internal" href = "restart.html" > < span class = "doc" > binary restart files< / span > < / a > . None of the < a class = "reference internal" href = "fix_modify.html" > < span class = "doc" > fix_modify< / span > < / a > options
are relevant to this fix. No global or per-atom quantities are stored
by this fix for access by various < span class = "xref std std-ref" > output commands< / span > . No parameter of this fix can
be used with the < em > start/stop< / em > keywords of the < a class = "reference internal" href = "run.html" > < span class = "doc" > run< / span > < / a > command.
This fix is not invoked during < a class = "reference internal" href = "minimize.html" > < span class = "doc" > energy minimization< / span > < / a > .< / p >
< / div >
< div class = "section" id = "restrictions" >
< h2 > Restrictions< / h2 >
< p > This fix style is part of the SHOCK package. It is only enabled if
LAMMPS was built with that package. See the < a class = "reference internal" href = "Section_start.html#start-3" > < span class = "std std-ref" > Making LAMMPS< / span > < / a > section for more info.< / p >
< p > 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.< / p >
< p > 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.< / p >
< / div >
< div class = "section" id = "related-commands" >
< h2 > Related commands< / h2 >
< p > < a class = "reference internal" href = "fix_wall.html" > < span class = "doc" > fix wall/reflect< / span > < / a > command, < a class = "reference internal" href = "fix_append_atoms.html" > < span class = "doc" > fix append/atoms< / span > < / a > command< / p >
< / div >
< div class = "section" id = "default" >
< h2 > Default< / h2 >
< p > The keyword defaults are pos = 0, vel = 0, units = lattice.< / p >
< / div >
< / div >
< / div >
< / div >
< footer >
< hr / >
< div role = "contentinfo" >
< p >
© Copyright 2013 Sandia Corporation.
< / p >
< / div >
Built with < a href = "http://sphinx-doc.org/" > Sphinx< / a > using a < a href = "https://github.com/snide/sphinx_rtd_theme" > theme< / a > provided by < a href = "https://readthedocs.org" > Read the Docs< / a > .
< / footer >
< / div >
< / div >
< / section >
< / div >
< script type = "text/javascript" >
var DOCUMENTATION_OPTIONS = {
URL_ROOT:'./',
VERSION:'',
COLLAPSE_INDEX:false,
FILE_SUFFIX:'.html',
HAS_SOURCE: true
};
< / script >
< script type = "text/javascript" src = "_static/jquery.js" > < / script >
< script type = "text/javascript" src = "_static/underscore.js" > < / script >
< script type = "text/javascript" src = "_static/doctools.js" > < / script >
< script type = "text/javascript" src = "https://cdn.mathjax.org/mathjax/latest/MathJax.js?config=TeX-AMS-MML_HTMLorMML" > < / script >
< script type = "text/javascript" src = "_static/sphinxcontrib-images/LightBox2/lightbox2/js/jquery-1.11.0.min.js" > < / script >
< script type = "text/javascript" src = "_static/sphinxcontrib-images/LightBox2/lightbox2/js/lightbox.min.js" > < / script >
< script type = "text/javascript" src = "_static/sphinxcontrib-images/LightBox2/lightbox2-customize/jquery-noconflict.js" > < / script >
< script type = "text/javascript" src = "_static/js/theme.js" > < / script >
< script type = "text/javascript" >
jQuery(function () {
SphinxRtdTheme.StickyNav.enable();
});
< / script >
< / body >
< / html >
