2007-10-11 06:39:11 +08:00
2015-07-30 22:53:28 +08:00
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< title > fix dt/reset command — LAMMPS 15 May 2015 version documentation< / title >
2007-10-11 06:39:11 +08:00
2015-07-30 22:53:28 +08:00
2007-10-11 06:39:11 +08:00
2015-07-30 22:53:28 +08:00
2007-10-11 06:39:11 +08:00
2015-07-30 22:53:28 +08:00
2007-10-11 06:39:11 +08:00
2015-07-30 22:53:28 +08:00
2007-10-11 06:39:11 +08:00
2015-07-30 22:53:28 +08:00
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< li > fix dt/reset command< / li >
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< span id = "index-0" > < / span > < h1 > fix dt/reset command< a class = "headerlink" href = "#fix-dt-reset-command" title = "Permalink to this headline" > ¶< / a > < / h1 >
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< 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 dt/reset N Tmin Tmax Xmax keyword values ...
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< ul class = "simple" >
< li > ID, group-ID are documented in < a class = "reference internal" href = "fix.html" > < em > fix< / em > < / a > command< / li >
< li > dt/reset = style name of this fix command< / li >
< li > N = recompute dt every N timesteps< / li >
< li > Tmin = minimum dt allowed which can be NULL (time units)< / li >
< li > Tmax = maximum dt allowed which can be NULL (time units)< / li >
< li > Xmax = maximum distance for an atom to move in one timestep (distance units)< / li >
< li > zero or more keyword/value pairs may be appended< / li >
< li > keyword = < em > units< / em > < / li >
< / ul >
< pre class = "literal-block" >
< em > units< / em > value = < em > lattice< / em > or < em > box< / em >
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lattice = Xmax is defined in lattice units
box = Xmax is defined in simulation box units
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< / pre >
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< 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 5 all dt/reset 10 1.0e-5 0.01 0.1
fix 5 all dt/reset 10 0.01 2.0 0.2 units box
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< div class = "section" id = "description" >
< h2 > Description< a class = "headerlink" href = "#description" title = "Permalink to this headline" > ¶< / a > < / h2 >
< p > Reset the timestep size every N steps during a run, so that no atom
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moves further than Xmax, based on current atom velocities and forces.
This can be useful when starting from a configuration with overlapping
atoms, where forces will be large. Or it can be useful when running
an impact simulation where one or more high-energy atoms collide with
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a solid, causing a damage cascade.< / p >
< p > This fix overrides the timestep size setting made by the
< a class = "reference internal" href = "timestep.html" > < em > timestep< / em > < / a > command. The new timestep size < em > dt< / em > is
computed in the following manner.< / p >
< p > For each atom, the timestep is computed that would cause it to
displace < em > Xmax< / em > on the next integration step, as a function of its
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current velocity and force. Since performing this calculation exactly
would require the solution to a quartic equation, a cheaper estimate
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is generated. The estimate is conservative in that the atom’ s
displacement is guaranteed not to exceed < em > Xmax< / em > , though it may be
smaller.< / p >
< p > Given this putative timestep for each atom, the minimum timestep value
across all atoms is computed. Then the < em > Tmin< / em > and < em > Tmax< / em > bounds are
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applied, if specified. If one (or both) is specified as NULL, it is
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not applied.< / p >
< p > When the < a class = "reference internal" href = "run_style.html" > < em > run style< / em > < / a > is < em > respa< / em > , this fix resets the
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outer loop (largest) timestep, which is the same timestep that the
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< a class = "reference internal" href = "timestep.html" > < em > timestep< / em > < / a > command sets.< / p >
< p > Note that the cumulative simulation time (in time units), which
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accounts for changes in the timestep size as a simulation proceeds,
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can be accessed by the < a class = "reference internal" href = "thermo_style.html" > < em > thermo_style time< / em > < / a > keyword.< / p >
< / div >
< 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 scalar which can be accessed by various
< a class = "reference internal" href = "Section_howto.html#howto-15" > < span > output commands< / span > < / a > . The scalar stores
the last timestep on which the timestep was reset to a new value.< / p >
< p > The scalar value calculated by this fix is “ intensive” .< / 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. This fix is not invoked during < a class = "reference internal" href = "minimize.html" > < em > energy minimization< / em > < / a > .< / 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 = "timestep.html" > < em > timestep< / em > < / a > < / p >
< / div >
< div class = "section" id = "default" >
< h2 > Default< a class = "headerlink" href = "#default" title = "Permalink to this headline" > ¶< / a > < / h2 >
< p > The option defaults is units = lattice.< / p >
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