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< li > fix viscous command< / li >
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< span id = "index-0" > < / span > < h1 > fix viscous command< a class = "headerlink" href = "#fix-viscous-command" title = "Permalink to this headline" > ¶< / a > < / h1 >
< / div >
< div class = "section" id = "fix-viscous-cuda-command" >
< h1 > fix viscous/cuda command< a class = "headerlink" href = "#fix-viscous-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 viscous gamma keyword values ...
< / pre > < / div >
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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 > viscous = style name of this fix command< / li >
< li > gamma = damping coefficient (force/velocity units)< / li >
< li > zero or more keyword/value pairs may be appended< / li >
< / ul >
< pre class = "literal-block" >
keyword = < em > scale< / em >
< em > scale< / em > values = type ratio
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type = atom type (1-N)
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ratio = factor to scale the damping coefficient by
< / 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 1 flow viscous 0.1
fix 1 damp viscous 0.5 scale 3 2.5
< / pre > < / div >
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< div class = "section" id = "description" >
< h2 > Description< a class = "headerlink" href = "#description" title = "Permalink to this headline" > ¶< / a > < / h2 >
< p > Add a viscous damping force to atoms in the group that is proportional
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to the velocity of the atom. The added force can be thought of as a
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frictional interaction with implicit solvent, i.e. the no-slip Stokes
drag on a spherical particle. In granular simulations this can be
useful for draining the kinetic energy from the system in a controlled
fashion. If used without additional thermostatting (to add kinetic
energy to the system), it has the effect of slowly (or rapidly)
freezing the system; hence it can also be used as a simple energy
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minimization technique.< / p >
< p > The damping force F is given by F = - gamma * velocity. The larger
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the coefficient, the faster the kinetic energy is reduced. If the
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optional keyword < em > scale< / em > is used, gamma can scaled up or down by the
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specified factor for atoms of that type. It can be used multiple
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times to adjust gamma for several atom types.< / p >
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< div class = "admonition note" >
< p class = "first admonition-title" > Note< / p >
< p class = "last" > You should specify gamma in force/velocity units. This is not
the same as mass/time units, at least for some of the LAMMPS
< a class = "reference internal" href = "units.html" > < em > units< / em > < / a > options like “ real” or “ metal” that are not
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self-consistent.< / p >
< / div >
< p > In a Brownian dynamics context, gamma = Kb T / D, where Kb =
Boltzmann’ s constant, T = temperature, and D = particle diffusion
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coefficient. D can be written as Kb T / (3 pi eta d), where eta =
dynamic viscosity of the frictional fluid and d = diameter of
particle. This means gamma = 3 pi eta d, and thus is proportional to
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the viscosity of the fluid and the particle diameter.< / p >
< p > In the current implementation, rather than have the user specify a
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viscosity, gamma is specified directly in force/velocity units. If
needed, gamma can be adjusted for atoms of different sizes
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(i.e. sigma) by using the < em > scale< / em > keyword.< / p >
< p > Note that Brownian dynamics models also typically include a randomized
force term to thermostat the system at a chosen temperature. The < a class = "reference internal" href = "fix_langevin.html" > < em > fix langevin< / em > < / a > command does this. It has the same
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viscous damping term as fix viscous and adds a random force to each
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atom. The random force term is proportional to the sqrt of the chosen
thermostatting temperature. Thus if you use fix langevin with a
target T = 0, its random force term is zero, and you are essentially
performing the same operation as fix viscous. Also note that the
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gamma of fix viscous is related to the damping parameter of < a class = "reference internal" href = "fix_langevin.html" > < em > fix langevin< / em > < / a > , however the former is specified in units
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of force/velocity and the latter in units of time, so that it can more
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easily be used as a thermostat.< / p >
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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 >
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< 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
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are relevant to this fix. No global or per-atom quantities are stored
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by this fix for access by various < a class = "reference internal" href = "Section_howto.html#howto-15" > < span > output commands< / span > < / a > . 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. This fix should only
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be used with damped dynamics minimizers that allow for
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non-conservative forces. See the < a class = "reference internal" href = "min_style.html" > < em > min_style< / em > < / a > command
for details.< / 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_langevin.html" > < em > fix langevin< / em > < / a > < / p >
< p > < strong > Default:< / strong > none< / p >
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