Hi, I need to compute the inverse of the moment of inertia (MOI) tensor of a bunch of point particles in a simulation algorithm. The number and location of the particles differs at each evaluation. In all cases, I'm taking the particle coordinates with respect to their center of mass. Everything works fine for n > 2 (n being the number of particles), but the n = 2 case is problematic. Using the usual MOI tensor formulas (http://en.wikipedia.org/wiki/Moment_of_inertia#Moment_of_inertia_tensor ), one can in fact show that the resulting MOI tensor for two particles, where the positions are defined with the origin set to the center of mass of the two particles, is singular (zero determinant; the algebra is messy, but the proof is straightforward). What am I doing wrong here? How can I get a non-singular MOI tensor for 2 particles?