.. index:: fix viscous

fix viscous command
===================

Syntax
""""""

.. parsed-literal::

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

* ID, group-ID are documented in :doc:`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

  .. parsed-literal::

     keyword = *scale*
       *scale* values = type ratio
         type = atom type (1-N)
         ratio = factor to scale the damping coefficient by

Examples
""""""""

.. code-block:: 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 :math:`F_i` is given by :math:`F_i = - \gamma v_i`.
The larger the coefficient, the faster the kinetic energy is reduced.
If the optional keyword *scale* is used, :math:`\gamma` can scaled up or
down by the specified factor for atoms of that type.  It can be used
multiple times to adjust :math:`\gamma` for several atom types.

.. 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
   :doc:`units <units>` options like "real" or "metal" that are not
   self-consistent.

In a Brownian dynamics context, :math:`\gamma = \frac{k_B T}{D}`, where
:math:`k_B =` Boltzmann's constant, *T* = temperature, and *D* = particle
diffusion coefficient.  *D* can be written as :math:`\frac{k_B T}{3 \pi
\eta d}`, where :math:`\eta =` dynamic viscosity of the frictional fluid
and d = diameter of particle.  This means :math:`\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, :math:`\gamma` is specified directly in force/velocity units.
If needed, :math:`\gamma` can be adjusted for atoms of different sizes
(i.e. :math:`\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
:doc:`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 :math:`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
:doc:`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.

----------

**Restart, fix_modify, output, run start/stop, minimize info:**

No information about this fix is written to :doc:`binary restart files
<restart>`.  None of the :doc:`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 :doc:`output commands <Howto_output>`.
No parameter of this fix can be used with the *start/stop* keywords of
the :doc:`run <run>` command.

The :doc:`fix_modify <fix_modify>` *respa* option is supported by this
fix. This allows to set at which level of the :doc:`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 :doc:`minimize <minimize>` command.  This fix should only
be used with damped dynamics minimizers that allow for
non-conservative forces.  See the :doc:`min_style <min_style>` command
for details.

Restrictions
""""""""""""
 none

Related commands
""""""""""""""""

:doc:`fix langevin <fix_langevin>`

**Default:** none
