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<CENTER><A HREF = "http://lammps.sandia.gov">LAMMPS WWW Site</A> - <A HREF = "Manual.html">LAMMPS Documentation</A> - <A HREF = "Section_commands.html#comm">LAMMPS Commands</A>
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<H3>fix viscous command
</H3>
<P><B>Syntax:</B>
</P>
<PRE>fix ID group-ID viscous gamma keyword values ...
</PRE>
<UL><LI>ID, group-ID are documented in <A HREF = "fix.html">fix</A> command
<LI>viscous = style name of this fix command
<LI>gamma = damping coefficient (force/velocity units)
<LI>zero or more keyword/value pairs can be appended
<LI>keyword = <I>b</I> or <I>a</I> or <I>t</I> or <I>m</I>
<LI>zero or more keyword/value pairs may be appended to the args
<PRE>keyword = <I>scale</I>
<I>scale</I> values = type ratio
type = atom type (1-N)
ratio = factor to scale the damping coefficient by
</PRE>
</UL>
<P><B>Examples:</B>
</P>
<PRE>fix 1 flow viscous 0.1
fix 1 damp viscous 0.5 scale 3 2.5
</PRE>
<P><B>Description:</B>
</P>
<P>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. 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 is a simple energy minimization
technique.
</P>
<P>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 <I>scale</I> 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.
</P>
<P>In a Brownian dynamics context, gamma = kT / mD, where k = Bolztmann's
constant, T = temperature, m = particle mass, and D = particle
diffusion coefficient. D can be written as kT / (6 pi eta d), where
eta = viscosity of the frictional fluid and d = diameter of particle.
This means gamma = 6 pi eta d, and thus is proportional to the
viscosity of the fluid and the particle diameter.
</P>
<P>In the current implementation, rather than have the user specify a
viscosity (in centiPoise or some other units), 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 <I>scale</I>
keyword.
</P>
<P>Note that Brownian dynamics models also typically include a randomized
force term to thermostat the system at a chosen temperature. The <A HREF = "fix_langevin.html">fix
langevin</A> command adds both a viscous damping term
and this random force to each atom; hence if using fix <I>langevin</I> you
do not typically need to use fix <I>viscous</I>.
</P>
<P><B>Restrictions:</B> none
</P>
<P><B>Related commands:</B>
</P>
<P><A HREF = "fix_langevin.html">fix langevin</A>
</P>
<P><B>Default:</B> none
</P>
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