forked from lijiext/lammps
121 lines
4.8 KiB
Plaintext
121 lines
4.8 KiB
Plaintext
"LAMMPS WWW Site"_lws - "LAMMPS Documentation"_ld - "LAMMPS Commands"_lc :c
|
|
|
|
:link(lws,http://lammps.sandia.gov)
|
|
:link(ld,Manual.html)
|
|
:link(lc,Section_commands.html#comm)
|
|
|
|
:line
|
|
|
|
fix viscous command :h3
|
|
fix viscous/cuda command :h3
|
|
|
|
[Syntax:]
|
|
|
|
fix ID group-ID viscous gamma keyword values ... :pre
|
|
|
|
ID, group-ID are documented in "fix"_fix.html command :ulb,l
|
|
viscous = style name of this fix command :l
|
|
gamma = damping coefficient (force/velocity units) :l
|
|
zero or more keyword/value pairs may be appended :l
|
|
keyword = {scale}
|
|
{scale} values = type ratio
|
|
type = atom type (1-N)
|
|
ratio = factor to scale the damping coefficient by :pre
|
|
:ule
|
|
|
|
[Examples:]
|
|
|
|
fix 1 flow viscous 0.1
|
|
fix 1 damp viscous 0.5 scale 3 2.5 :pre
|
|
|
|
[Description:]
|
|
|
|
Add a viscous damping force to atoms in the group that is proportional
|
|
to the velocity of the atom. The added force can be thought of as a
|
|
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
|
|
minimization technique.
|
|
|
|
The damping force F is given by F = - gamma * velocity. The larger
|
|
the coefficient, the faster the kinetic energy is reduced. If the
|
|
optional keyword {scale} is used, gamma can scaled up or down by the
|
|
specified factor for atoms of that type. It can be used multiple
|
|
times to adjust gamma for several atom types.
|
|
|
|
IMPORTANT NOTE: You should specify gamma in force/velocity units.
|
|
This is not the same as mass/time units, at least for some of the
|
|
LAMMPS "units"_units.html options like "real" or "metal" that are not
|
|
self-consistent.
|
|
|
|
In a Brownian dynamics context, gamma = Kb T / D, where Kb =
|
|
Boltzmann's constant, T = temperature, and D = particle diffusion
|
|
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
|
|
the viscosity of the fluid and the particle diameter.
|
|
|
|
In the current implementation, rather than have the user specify a
|
|
viscosity, gamma is specified directly in force/velocity units. If
|
|
needed, gamma can be adjusted for atoms of different sizes
|
|
(i.e. sigma) by using the {scale} keyword.
|
|
|
|
Note that Brownian dynamics models also typically include a randomized
|
|
force term to thermostat the system at a chosen temperature. The "fix
|
|
langevin"_fix_langevin.html command does this. It has the same
|
|
viscous damping term as fix viscous and adds a random force to each
|
|
atom. Hence if using fix {langevin} you do not typically need to use
|
|
fix {viscous}. Also note that the gamma of fix viscous is related to
|
|
the damping parameter of "fix langevin"_fix_langevin.html, except that
|
|
the units of gamma are force/velocity and the units of damp are time,
|
|
so that it can more easily be used as a thermostat.
|
|
|
|
:line
|
|
|
|
Styles with a {cuda} suffix are functionally the same as the
|
|
corresponding style without the suffix. They have been optimized to
|
|
run faster, depending on your available hardware, as discussed in
|
|
"this section"_Section_accelerate.html of the manual. The accelerated
|
|
styles take the same arguments and should produce the same results,
|
|
except for round-off and precision issues.
|
|
|
|
These accelerated styles are part of the "user-cuda" package. They
|
|
are only enabled if LAMMPS was built with that package. See the
|
|
"Making LAMMPS"_Section_start.html#2_3 section for more info.
|
|
|
|
You can specify the accelerated styles explicitly in your input script
|
|
by including their suffix, or you can use the "-suffix command-line
|
|
switch"_Section_start.html#2_6 when you invoke LAMMPS, or you can use
|
|
the "suffix"_suffix.html command in your input script.
|
|
|
|
See "this section"_Section_accelerate.html of the manual for more
|
|
instructions on how to use the accelerated styles effectively.
|
|
|
|
:line
|
|
|
|
[Restart, fix_modify, output, run start/stop, minimize info:]
|
|
|
|
No information about this fix is written to "binary restart
|
|
files"_restart.html. None of the "fix_modify"_fix_modify.html options
|
|
are relevant to this fix. No global or per-atom quantities are stored
|
|
by this fix for access by various "output
|
|
commands"_Section_howto.html#4_15. No parameter of this fix can be
|
|
used with the {start/stop} keywords of the "run"_run.html command.
|
|
|
|
The forces due to this fix are imposed during an energy minimization,
|
|
invoked by the "minimize"_minimize.html command. This fix should only
|
|
be used with damped dynamics minimizers that allow for
|
|
non-conservative forces. See the "min_style"_min_style.html command
|
|
for details.
|
|
|
|
[Restrictions:] none
|
|
|
|
[Related commands:]
|
|
|
|
"fix langevin"_fix_langevin.html
|
|
|
|
[Default:] none
|