</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.

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.

</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

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 = Boltzmann'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>In a Brownian dynamics context, gamma = kT / D, where k = Boltzmann's

constant, T = temperature, and D = particle diffusion coefficient.  D

can be written as kT / (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.

</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.

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 <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>.

langevin</A> command does this.  It has the same

viscous damping term as fix viscous and adds a random force to each

atom.  Hence if using fix <I>langevin</I> you do not typically need to use

fix <I>viscous</I>.  Also note that the gamma of fix viscous is related to

the damping parameter of <A HREF = "fix_langevin.html">fix langevin</A>, except that

the units of gamma are force/velocity (or mass/time) and the units of

damp are time, so that it can more easily be used as a thermostat.

</P>

<P><B>Restart, fix_modify, output, run start/stop, minimize info:</B>

</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.

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

specified factor for atoms of that type.  It can be used multiple

times to adjust gamma for several atom types.

In a Brownian dynamics context, gamma = kT / mD, where k = Boltzmann'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.

In a Brownian dynamics context, gamma = kT / D, where k = Boltzmann's

constant, T = temperature, and D = particle diffusion coefficient.  D

can be written as kT / (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 (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 {scale}

keyword.

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.html command adds both a viscous damping term

and this random force to each atom; hence if using fix {langevin} you

do not typically need to use fix {viscous}.

langevin"_fix_langevin.html command does this.  It has the same

viscous damping term as fix viscous and adds a random force to each

atom.  Hence if using fix {langevin} you do not typically need to use

fix {viscous}.  Also note that the gamma of fix viscous is related to

the damping parameter of "fix langevin"_fix_langevin.html, except that

the units of gamma are force/velocity (or mass/time) and the units of

damp are time, so that it can more easily be used as a thermostat.

[Restart, fix_modify, output, run start/stop, minimize info:]