Re: [ESPResSo-users] Educated guess about gamma in Langevin thermostat

[Stefan Kesselheim]

Dear everybody,
sorry, I assumed, you wanted to use LB.
But it seems you don't, and the Ulf is right :-). And you should 
remember: High friction makes all processes slow, including relaxation 
towards thermodynamic equilibrium.
Cheers
Stefan

On 01/14/2013 10:32 AM, Stefan Kesselheim wrote:
> Hi,
> I have one very small remark:
> The combination of viscosity and (apparent -- see Ahlrichs 98) 
> friction determines the effective size of the particles. The radius 
> acts as the prefactor of the hydrodynamic interactions (see e.g. 
> literature on Stokesean Dynamics). Sometimes this length scale 
> (compared to other length scales in the system) must be matched. 
> Especially in low Re conditions matching this can sufficient, as long 
> as the Schmidt number is high enough. The hydrodynamic particle size 
> remains constant as long as visc/friction is held constant, and it can 
> not be much larger than the LB lattice size (this is because the 
> coupling is point--like).
> Cheers
> Stefan
>
> On 01/11/2013 05:40 PM, Ulf Schiller wrote:
> > Hi Salvador,
> >
> > On 12/19/2012 06:34 PM, Salvador H-V wrote:
> > > I am doing some simulations for a two-dimensional  hard-sphere
> > > rigig-dumbbells.
> > > The interaction potential is the purely repulsive Lennard-Jones (WCA)
> > > using the rigid_bond feature to constraint the bond in the dimers.
> > >
> > > I would like to choose a value  of the the friction coefficient 
> > > (gamma =
> > > 6.0*Pi*dynamic_viscosity*sphere_radius / mass  )  in the Langevin 
> > > thermostat
> > > such as is representative of the solvent experimental viscosity.
> > >
> > > Using the experimental data, I obtain the following
> > > M ~ 4.4x10^(-15) kg
> > > T ~ 293.15 K
> > > tao = sigma * ( mass / kbT)^1/2 ~ 2.08x10^-3 s
> > > viscosity ~ 0.001002 Pa * s
> > > sigma = 2x10^-6 m
> > >
> > > Then, gamma_langevin = 6 * Pi * viscosity * radius / mass
> > > and in reduced units  gamma / tao = gamma_reduced ~ 8930
> > >
> > > If my above simple calculations are right and if I understood well,
> > > accordingly to previous post in the mail list,  we have to use a value
> > > of time_step
> > > such as:  gamma_reduced * dt  / 2.0 is < 1 and preferably around  0.1.
> > >
> > > Then, I should use a time_step <= 0.00002 that is very small and will
> > > require very long simulations to obtain the experimental time window.
> > >
> > > I was wondering if somebody could provide suggestions of how to reduce
> > > the value of gamma (so, i can increase the time_step) but still
> > > representing the solvent viscosity.
> >
> > I think that depends on what physics you want to look at. If inertial 
> > effects can be neglected in your system, you could try to use an 
> > artificial mass instead of matching the experimental mass. Keep in 
> > mind that a too high value of gamma will also affect the accuracy of 
> > the integrator (the Verlet algorithm with velocity dependent forces 
> > is first order only).
> >
> > Another possibility is to think whether you really need the exact 
> > value of the viscosity. It might be enough to match dimensionless 
> > quantities such as the Reynolds number, Peclet number, etc. One can 
> > then typically scale the simulation parameters to obtain reasonable 
> > values. The details depend again on what you want to simulate/measure.
> >
> > Hope this helps,
> > Ulf