Warp wrote:

>   But you have to calculate the center of mass of the whole group, and
> since each element of the group can change independently of the others,
> that would mean you have to re-calculate this center of mass each time
> anything changes.

That's true. I think you're probably still saving yourself work though. 
Compute that center of mass once and use it many times, verses computer 
many object-object interactions many times over.

It depends on what you're simulating of course, but I would think there 
are probably also situations were you can just disregard really distant 
particles.

>   Also another problem is that groups don't stay the same. If a particle
> not belonging to the group enters the "inside" of the group, it cannot
> be calculated anymore against that group as if it was just one single
> mass located at the center of mass of the group.

Isn't this just the "normal" problem of performing space partitioning 
with moving objects?

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