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Re: [ESPResSo-users] Warning Message in LB

From: Axel Arnold
Subject: Re: [ESPResSo-users] Warning Message in LB
Date: Thu, 05 Jun 2014 23:33:09 +0200
User-agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.9; rv:24.0) Gecko/20100101 Thunderbird/24.5.0

Dear Mohamed,

please use our mailing list for Espresso related questions.

On 05.06.14 15:23, s1mokhal wrote:
Dear Mr Arnold,

My name is Mohamed Abdelkhalek, currently working on a bachelor thesis project in Bayreuth University involving forking a version of the latest espresso code and adding a few things for red blood cell simulations in lb fluid. Anyway, I am having trouble understanding this warning

"Recalculating forces, so the LB coupling forces are not included in the particle force the first time step. This only matters if it happens frequently during sampling."

As it keeps appearing repeatedly in my simulations which eventually break down. So, I hoped if you can explain the exact meaning of the warning and it's consequences ? for example when using multiple processors I see the warning all the time.

The message says that the recalc_forces flag has been set by some algorithm due to changes that you made between two the integrate commands. That can be anything from changing interactions to changing particles. If you do two integrate commands one after the other without any other command inbetween, this should not happen, apart from your own modifications.

As the warning says, there will be no particle coupling forces for the first (half) time step, that is, particles will not feel the fluid. This disturbs the LB quite a bit, and you should not use results obtained with this message appearing regularly.

If you are sure that there is no need to recalculate forces, then you can use the "reuse_forces" option of the integrate command, which I added recently, and enforces reuse. But that of course means that any changes to interactions or particles do _not_ apply for the first half time step.

Finally, if you are only embedding blood cells, there is no need to worry, as the blood cell marker particles are propaged differently. Just if you also embed conventional particles, you are in trouble.


JP Dr. Axel Arnold
ICP, Universit├Ąt Stuttgart
Allmandring 3
70569 Stuttgart, Germany
Email: address@hidden
Tel: +49 711 685 67609

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