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< title > fix aveforce command — LAMMPS documentation< / title >
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< li class = "toctree-l1" > < a class = "reference internal" href = "Section_intro.html" > 1. Introduction< / a > < / li >
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< li > fix aveforce command< / li >
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< span id = "index-0" > < / span > < h1 > fix aveforce command< a class = "headerlink" href = "#fix-aveforce-command" title = "Permalink to this headline" > ¶< / a > < / h1 >
< / div >
< div class = "section" id = "fix-aveforce-cuda-command" >
< h1 > fix aveforce/cuda command< a class = "headerlink" href = "#fix-aveforce-cuda-command" title = "Permalink to this headline" > ¶< / a > < / h1 >
< div class = "section" id = "syntax" >
< h2 > Syntax< a class = "headerlink" href = "#syntax" title = "Permalink to this headline" > ¶< / a > < / h2 >
< div class = "highlight-python" > < div class = "highlight" > < pre > fix ID group-ID aveforce fx fy fz keyword value ...
< / pre > < / div >
< / div >
< ul class = "simple" >
< li > ID, group-ID are documented in < a class = "reference internal" href = "fix.html" > < em > fix< / em > < / a > command< / li >
< li > aveforce = style name of this fix command< / li >
< li > fx,fy,fz = force component values (force units)< / li >
< / ul >
< div class = "highlight-python" > < div class = "highlight" > < pre > any of fx,fy,fz can be a variable (see below)
< / pre > < / div >
< / div >
< ul class = "simple" >
< li > zero or more keyword/value pairs may be appended to args< / li >
< li > keyword = < em > region< / em > < / li >
< / ul >
< pre class = "literal-block" >
< em > region< / em > value = region-ID
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region-ID = ID of region atoms must be in to have added force
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< / pre >
< / div >
< div class = "section" id = "examples" >
< h2 > Examples< a class = "headerlink" href = "#examples" title = "Permalink to this headline" > ¶< / a > < / h2 >
< div class = "highlight-python" > < div class = "highlight" > < pre > fix pressdown topwall aveforce 0.0 -1.0 0.0
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fix 2 bottomwall aveforce NULL -1.0 0.0 region top
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fix 2 bottomwall aveforce NULL -1.0 v_oscillate region top
< / pre > < / div >
< / div >
< / div >
< div class = "section" id = "description" >
< h2 > Description< a class = "headerlink" href = "#description" title = "Permalink to this headline" > ¶< / a > < / h2 >
< p > Apply an additional external force to a group of atoms in such a way
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that every atom experiences the same force. This is useful for
pushing on wall or boundary atoms so that the structure of the wall
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does not change over time.< / p >
< p > The existing force is averaged for the group of atoms, component by
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component. The actual force on each atom is then set to the average
value plus the component specified in this command. This means each
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atom in the group receives the same force.< / p >
< p > Any of the fx,fy,fz values can be specified as NULL which means the
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force in that dimension is not changed. Note that this is not the
same as specifying a 0.0 value, since that sets all forces to the same
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average value without adding in any additional force.< / p >
< p > Any of the 3 quantities defining the force components can be specified
as an equal-style < a class = "reference internal" href = "variable.html" > < em > variable< / em > < / a > , namely < em > fx< / em > , < em > fy< / em > , < em > fz< / em > .
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If the value is a variable, it should be specified as v_name, where
name is the variable name. In this case, the variable will be
evaluated each timestep, and its value used to determine the average
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force.< / p >
< p > Equal-style variables can specify formulas with various mathematical
functions, and include < a class = "reference internal" href = "thermo_style.html" > < em > thermo_style< / em > < / a > command
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keywords for the simulation box parameters and timestep and elapsed
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time. Thus it is easy to specify a time-dependent average force.< / p >
< p > If the < em > region< / em > keyword is used, the atom must also be in the
specified geometric < a class = "reference internal" href = "region.html" > < em > region< / em > < / a > in order to have force added
to it.< / p >
< hr class = "docutils" / >
< p > Styles with a < em > cuda< / em > suffix are functionally the same as the
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corresponding style without the suffix. They have been optimized to
run faster, depending on your available hardware, as discussed in
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< a class = "reference internal" href = "Section_accelerate.html" > < em > Section_accelerate< / em > < / a > of the manual. The
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accelerated styles take the same arguments and should produce the same
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results, except for round-off and precision issues.< / p >
< p > These accelerated styles are part of the USER-CUDA package. They are
only enabled if LAMMPS was built with that package. See the < a class = "reference internal" href = "Section_start.html#start-3" > < span > Making LAMMPS< / span > < / a > section for more info.< / p >
< p > You can specify the accelerated styles explicitly in your input script
by including their suffix, or you can use the < a class = "reference internal" href = "Section_start.html#start-7" > < span > -suffix command-line switch< / span > < / a > when you invoke LAMMPS, or you can
use the < a class = "reference internal" href = "suffix.html" > < em > suffix< / em > < / a > command in your input script.< / p >
< p > See < a class = "reference internal" href = "Section_accelerate.html" > < em > Section_accelerate< / em > < / a > of the manual for
more instructions on how to use the accelerated styles effectively.< / 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< a class = "headerlink" href = "#restart-fix-modify-output-run-start-stop-minimize-info" title = "Permalink to this headline" > ¶< / a > < / h2 >
< p > No information about this fix is written to < a class = "reference internal" href = "restart.html" > < em > binary restart files< / em > < / a > . None of the < a class = "reference internal" href = "fix_modify.html" > < em > fix_modify< / em > < / a > options
are relevant to this fix.< / p >
< p > This fix computes a global 3-vector of forces, which can be accessed
by various < a class = "reference internal" href = "Section_howto.html#howto-15" > < span > output commands< / span > < / a > . This is the
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total force on the group of atoms before the forces on individual
atoms are changed by the fix. The vector values calculated by this
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fix are “ extensive” .< / p >
< p > No parameter of this fix can be used with the < em > start/stop< / em > keywords of
the < a class = "reference internal" href = "run.html" > < em > run< / em > < / a > command.< / p >
< p > The forces due to this fix are imposed during an energy minimization,
invoked by the < a class = "reference internal" href = "minimize.html" > < em > minimize< / em > < / a > command. You should not
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specify force components with a variable that has time-dependence for
use with a minimizer, since the minimizer increments the timestep as
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the iteration count during the minimization.< / p >
< / div >
< div class = "section" id = "restrictions" >
< h2 > Restrictions< a class = "headerlink" href = "#restrictions" title = "Permalink to this headline" > ¶< / a > < / h2 >
< blockquote >
< div > none< / div > < / blockquote >
< / div >
< div class = "section" id = "related-commands" >
< h2 > Related commands< a class = "headerlink" href = "#related-commands" title = "Permalink to this headline" > ¶< / a > < / h2 >
< p > < a class = "reference internal" href = "fix_setforce.html" > < em > fix setforce< / em > < / a > , < a class = "reference internal" href = "fix_addforce.html" > < em > fix addforce< / em > < / a > < / p >
< p > < strong > Default:< / strong > none< / p >
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© Copyright 2013 Sandia Corporation.
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