[MMTK] Bug in MMTK.Universe.largestDistance()?

Konrad Hinsen konrad.hinsen at laposte.net
Mon Jan 7 09:10:51 UTC 2008

On 04.01.2008, at 13:11, Andreas Kring wrote:

> I think there is a bug in MMTK.Universe.largestDistance(). An  
> example of
> the error can be seen in the script below.

There is indeed a bug, but it is not where you think it is. The  
calculation for orthogonal universes (including cubic ones) is  
correct, but the one for non-orthogonal boxes isn't.

What largestDistance() should return is the largest interparticle  
distance that can be represented in the universe independent of  
orientation. In other words, the largest distance L is defined as the  
largest L such that an atom A2 placed at a distance L from a given  
atom A1 in an arbitrarily chosen direction will
make the minimum-image distance between A1 and A2 equal to L. For a  
rectangular box, this makes L equal to half the shortest edge length.

This method is used in MMTK for checking cutoff radii for particle  
interactions. For consistency, all real-space cutoffs should be less  
than the number returned by universe.largestDistance(). For any  
larger cutoff, the effective interaction volume of each atom is no  
longer spherical.

Konrad Hinsen
Centre de Biophysique Moléculaire, CNRS Orléans
Synchrotron Soleil - Division Expériences
Saint Aubin - BP 48
91192 Gif sur Yvette Cedex, France
Tel. +33-1 69 35 97 15
E-Mail: hinsen at cnrs-orleans.fr

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