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

From: Salvador H-V
Subject: [ESPResSo-users] Educated guess about gamma in Langevin thermostat
Date: Wed, 19 Dec 2012 11:34:24 -0600

Dear All,

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. 

Any help/suggestion or reference would be greatly appreciated.


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