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[Bruno Chareyre <bruno.chareyre@xxxxxxxxxxx>]

This might be of interest for us :

"Quaternion-based rigid body rotation integration algorithms for use in 
particle methods"
JOHNSON Scott M.^ (1) ; WILLIAMS John R.^ (2) ; COOK Benjamin K.^ (3) ;
International journal for numerical methods in engineering
2008, vol. 74, n^o 8, pp. 1303-132

Abstract:
The resolution of translational motion in discrete element method and 
molecular dynamics applications is a straightforward task; however, 
resolving rotational motion is less obvious. Many applications update 
rotation using an explicit integration involving products of matrices, 
which has well-known drawbacks. Although rigid body rotation has 
received attention in large-angle rotation applications, relatively 
little attention has been dedicated to the unique requirements of 
particle methods using explicit time-stepping algorithms. This paper 
reviews existing explicit algorithms and shows the benefits of using a 
quaternion-based re-parameterization of both the central difference 
algorithm and the approach of Munjiza et al. (Int. J. Numer. Meth. Engng 
2003; 56:36-55). The improvement not only provides guaranteed 
orthonormality of the resulting rotation but also allows the assumption 
of small-angle rotation to be relaxed and the use of a more accurate 
Taylor expansion instead. The current and quaternion-based algorithms 
are compared for accuracy and computational efficiency.

Bruno


p.s. I can send pdf copies in private mail if you don't have access to 
this journal.




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