2006-09-22 00:22:34 +08:00
|
|
|
<HTML>
|
|
|
|
|
<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>
|
|
|
|
|
</CENTER>
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
<HR>
|
|
|
|
|
|
|
|
|
|
<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>
|
|
|
|
|
|
2007-07-03 04:04:44 +08:00
|
|
|
<LI>zero or more keyword/value pairs may be appended
|
2006-09-22 00:22:34 +08:00
|
|
|
|
|
|
|
|
<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>
|
2007-10-11 06:28:11 +08:00
|
|
|
<P><B>Restart, fix_modify, output, run start/stop, minimize info:</B>
|
2007-06-26 08:03:39 +08:00
|
|
|
</P>
|
|
|
|
|
<P>No information about this fix is written to <A HREF = "restart.html">binary restart
|
|
|
|
|
files</A>. None of the <A HREF = "fix_modify.html">fix_modify</A> options
|
2007-10-11 06:28:11 +08:00
|
|
|
are relevant to this fix. No global scalar or vector or per-atom
|
|
|
|
|
quantities are stored by this fix for access by various <A HREF = "Section_howto.html#4_15">output
|
|
|
|
|
commands</A>. No parameter of this fix can be
|
|
|
|
|
used with the <I>start/stop</I> keywords of the <A HREF = "run.html">run</A> command.
|
|
|
|
|
This fix is not invoked during <A HREF = "minimize.html">energy minimization</A>.
|
2007-06-26 08:03:39 +08:00
|
|
|
</P>
|
2006-09-22 00:22:34 +08:00
|
|
|
<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>
|
|
|
|
|
</HTML>
|
