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Re: [ESPResSo] fixing center of mass

From: gil claudio
Subject: Re: [ESPResSo] fixing center of mass
Date: Wed, 22 Oct 2008 00:30:27 -0700 (PDT)

Hi Axel,

It worked.  Thanks.


----- Original Message ----
From: Axel Arnold <address@hidden>
To: address@hidden
Sent: Monday, October 20, 2008 10:04:24 AM
Subject: Re: [ESPResSo] fixing center of mass

Am Freitag 17 Oktober 2008 14:06 schrieb gil claudio:
> How does one fix the center of mass of a group of particles at a specific
> point, given that the simulation uses periodic boundary conditions?
> Here is how I did it:
> I placed the center of my group of particles (type 1) at the point I wanted
> it fixed, then called inter 1 1 comfixed 1
> I got this error message:
> works only with non periodic BC


well, that is hardcoded in comfixed.h. However, I am not quite sure why, since 
as far as I understand it, the routine in general does not depend on the 
boundary conditions; it simply shifts the forces such that the force on the 
COM is zero, which also works under PBC. Also, I do not quite see why setting 
the comfixed-interaction actually zeros the forces and velocities of the 
fixed particles.

Does anybody know why?

In the meantime, I would suggest you simply remove the check for the boundary 
conditions (lines 39-41) and see whether it then works as you expect.


Dr. Axel Arnold 
Fraunhofer SCAI
Schloss Birlinghoven, 53754 Sankt Augustin, Germany
Tel: +49 2241 14 2575

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