# fix viscous command

Accelerator Variants: *viscous/kk*

## Syntax

    fix ID group-ID viscous gamma keyword values ...

-   ID, group-ID are documented in [fix](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

``` LAMMPS
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 $F_i$ is given by $F_i = - \gamma v_i$. 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.

:::: note
::: title
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)
options like \"real\" or \"metal\" that are not self-consistent.
::::

In a Brownian dynamics context, $\gamma = \frac{k_B T}{D}$, where
$k_B =$ Boltzmann\'s constant, $T$ = temperature, and *D* = particle
diffusion coefficient. *D* can be written as $\frac{k_B
T}{3 \pi \eta d}$, 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) 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
$T = 0$, its random force term is zero, and you are essentially
performing the same operation as fix viscous. Also note that the gamma
of fix viscous is related to the damping parameter of [fix
langevin](fix_langevin), however the former is specified in units of
force/velocity and the latter in units of time, so that it can more
easily be used as a thermostat.

------------------------------------------------------------------------

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](Speed_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](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](Run_options) when you invoke LAMMPS, or you can use the
[suffix](suffix) command in your input script.

See the [Accelerator packages](Speed_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](restart). None of the [fix_modify](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](Howto_output). No
parameter of this fix can be used with the *start/stop* keywords of the
[run](run) command.

The [fix_modify](fix_modify) *respa* option is supported by this fix.
This allows to set at which level of the [r-RESPA](run_style) 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](minimize) command. This fix should only be
used with damped dynamics minimizers that allow for non-conservative
forces. See the [min_style](min_style) command for details.

## Restrictions

none

## Related commands

[fix langevin](fix_langevin), [fix viscous/sphere](fix_viscous_sphere),
[fix damping/cundall](fix_damping_cundall)

## Default

none