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Re: [ESPResSo] fixing center of mass

From: gil claudio
Subject: Re: [ESPResSo] fixing center of mass
Date: Wed, 5 Nov 2008 22:47:05 -0800 (PST)

Hi Mehmet,

Luckily the particles whose COM I would like to constrain are all bonded to one 

Thanks for clarifying this issue.


----- Original Message ----
From: Mehmet Sayar <address@hidden>
To: Axel Arnold <address@hidden>
Cc: address@hidden; address@hidden
Sent: Wednesday, November 5, 2008 4:24:48 PM
Subject: Re: [ESPResSo] fixing center of mass

Hi Gil and Axel,

I just saw this email about the comfixed algorithm.

The noPBC condition was put in to the code for the following reason:
Imagine that you want to fix the center of mass of a group of particles
that are not covalently bonded. They are just held together via some nonbonded
potentials. If for some reason the aggregate falls apart and some particles
cross the boundaries, in that case the COM of the aggregate becomes meaningless.

I agree that this is a rather special case. So if you want we can remove the
noPBC condition from the code, and just put a warning into the documentation.


Axel Arnold wrote:
> 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
> Hi,
> 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.
> Axel

Mehmet Sayar - Assist. Prof.  Koc University
Email:address@hidden        Dept. Mechanical Eng.
Phone:+90-212-338-1840        Rumelifeneri Yolu, Sariyer
Fax:  +90-212-338-1548        34450 Istanbul, Turkey


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