Re: [ESPResSo-users] DPD

[Ulf Schiller]

On 11/09/14 15:20, Stefan Kesselheim wrote:
> Dear Dusan,
> 
> On Sep 11, 2014, at 4:07 PM, Dudo <address@hidden> wrote:
> 
>> Dear Jens,
>>
>> thank you very much for the lead.
>>
>> Well, so I've compiled in a feature for "TUNABLE_SLIP"
>> and I have set up interactions: inter $cid $idpolym tunable_slip 
>> $temperature $gamma_L $r_cut_L $timestep .....
>> I have turned the flag to 2: constraint cylinder center $cx $cy $cz axis 
>> $cnx $cny $cnz radius $crad length $clength direction $cdirection type $cid 
>> reflecting 2 .....
>> as Stefan suggested, and I'm trying with the parameters from your paper on 
>> polyelectrolytes J. Phys Chem B 2010..
>> Now I see you have used explicit solvent with density 3.75, while modelling 
>> a chain of 20 beads.
>> In my case this would mean zillions of solvent particles..
>> Well at the begining Chris asked me, why would I do add explicit particles - 
>> so.. is there a way around?
> 
> The particle numbers that you will need are necessary are considerable but 
> OK. For a chain of 20 beads, the radius of gyration is probably around 5, and 
> thus you need a box size of 10, and 20 is still doable. With a density of 1, 
> you'll have 1k to 8k particles. That should be enough to perform the 
> measurements, especially as you probably don't need too many steps to get a 
> meaningful mobility. 
> 
> In principle you can use even lower densities, but at some point the mean 
> free path of the particles becomes comparable to the box size, and then you 
> are in a region where you should be sure what you are doing.

Keep in mind that polymeric properties are strongly affected by the
solvent self-diffusion and make sure the Schmidt number is reasonable.
Further details in http://dx.doi.org/10.1063/1.2360274 which also
contains remarks on the slip flow model.

Cheers,
Ulf

-- 
Dr Ulf D Schiller
Centre for Computational Science
University College London
20 Gordon Street
London WC1H 0AJ
United Kingdom