fix viscous command
Accelerator Variants: viscous/kk
Syntax
fix ID group-ID viscous gamma keyword values ...
ID, group-ID are documented in fix command
viscous = style name of this fix command
gamma = damping coefficient (force/velocity units)
zero or more keyword/value pairs may be appended
keyword = scale scale values = type ratio type = atom type (1-N) ratio = factor to scale the damping coefficient by
Examples
fix 1 flow viscous 0.1
fix 1 damp viscous 0.5 scale 3 2.5
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
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 options like “real” or “metal” that are not self-consistent.
In a Brownian dynamics context,
In the current implementation, rather than have the user specify a
viscosity,
Note that Brownian dynamics models also typically include a randomized
force term to thermostat the system at a chosen temperature. The
fix langevin command does this. It has the same
viscous damping term as fix viscous and adds a random force to each
atom. The random force term is proportional to the square root of the
chosen thermostatting temperature. Thus if you use fix langevin with a
target
Styles with a gpu, intel, kk, omp, or opt 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 on the Accelerator packages page. 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 GPU, INTEL, KOKKOS, OPENMP, and OPT packages, respectively. They are only enabled if LAMMPS was built with those packages. See the Build package page 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 when you invoke LAMMPS, or you can use the suffix command in your input script.
See the Accelerator packages page for more instructions on how to use the accelerated styles effectively.
Restart, fix_modify, output, run start/stop, minimize info
No information about this fix is written to binary restart files. None of the fix_modify options are relevant to this fix. No global or per-atom quantities are stored by this fix for access by various output commands. No parameter of this fix can be used with the start/stop keywords of the run command.
The fix_modify respa option is supported by this fix. This allows to set at which level of the r-RESPA integrator the fix is modifying forces. Default is the outermost level.
The forces due to this fix are imposed during an energy minimization, invoked by the minimize command. This fix should only be used with damped dynamics minimizers that allow for non-conservative forces. See the min_style command for details.
Restrictions
none
Default
none