# fix viscous command

## 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 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.

Warning

You should specify gamma in force/velocity units. This is not the same as mass/time units, at least for some of the LIGGGHTS(R)-PUBLIC units 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` 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 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 gamma of fix viscous is related to the damping parameter of `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.

## 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 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

## Related commands

`fix langevin`

Default: none