Re: [ESPResSo] Questions about Thermostat

[Christoph Junghans]

Dear Lozenro,

Torsten's answer was not 100% correct. There is the (so far 
undocumented) inter_dpd, which allows you to use different frictions for 
different particle types.

######### BEGIN technical details#######
use #define INTER_DPD in myconfig.h
set thermostat inter_dpd TEMP
inter X Y inter_dpd GAMMA CUTOFF WF TGAMMA TCUTOFF TWF
######### END technical details#########

The only problem here is: How do distinguish between different 
aggregates? By particle type?

Because inter_dpd uses particle types to decide what to do.
(Btw. changing the particle type of fly is not a good idea for mpi.)

Cheers,

Christoph

Lorenzo Isella schrieb:
> Hello Torsten,
> And thanks for your reply.
> > > (2) In the simulations I ran, I start with a certain number of
> > > monomers (i.e. the initial particles I place in the computational box)
> > > which stick upon collisions thus giving rise to aggregates.
> > > The diffusion coefficient of the resulting aggregate does not take
> > > into account the fact that a monomer deep inside the aggregate is
> > > "screened" by the surrounding monomers and hence feels a different
> > > noise with respect to a "free" Brownian monomer.
> > > I wonder whether the DPD thermostat in Espresso can somehow account
> > > for that, but I am not sure it is a reasonable choice and would like
> > > to know the opinion of someone on this list.
> > I'm not sure, if I understood your question correctly. Do you want to
> > apply different DPD prefactors (gamma and temperature) to particles
> > whether they are inside or outside of aggregates ?
>
> Not exactly.  Let us say that you have two aggregates each made up of 50 
> monomers (think of each aggregate as a structure  consisting of 50 
> spherules).
> Aggregate A has a different shape than aggregate B (A almost looks like 
> a sphere whereas B is elongated, chain-like).
> Physically, you would expect them to diffuse differently (in a quiescent 
> fluid). The reason is that each monomer in aggregate A "feels" 
> differently the fluid than in aggregate B.  For instance, the inner 
> monomers in aggregate B have a very small surface directly exposed to 
> the fluid.
> But if I study their diffusion properties using Langevin thermostat 
> (hence using Langevin equation), in the end I see no difference, the 
> reason being that nothing in Langevin equation accounts for the 
> screening of the inner monomers in an aggregate.
> Notice that I am not using the thermostat to model the coupling of the 
> system with a thermal bath, but to investigate its coarse-grained 
> dynamics (mesoscopic rather than microscopic description).
> Now, I wonder if the DPD thermostat can do what I need (which is not to 
> model shear at this stage, but to make each monomer aware of the 
> surrounding monomers within the aggregate, which should affect the 
> aggregate diffusion properties).
> Hope this clarifies what I am trying to do.
> Cheers
>
> Lorenzo
>
>  
>
>
>
>
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--
Dipl.-Phys. Christoph Junghans
Max Planck Institute for Polymer Research
Theory Group
POBox 3148
D 55021 Mainz, Germany

Phone: +49 6131 379 335
Web: http://www.mpip-mainz.mpg.de/~junghans