Hi Tom, I saw your message on the web and I tried to use your script on Chimera 1.3. Unfortunately it does not work, it give me the following message: 0 models opened Do you think it is because of the Chimera version? Do you have suggestion? Thank you very much, Armida Oops. That script I posted for calculating the inertia of a molecule had some problems. The printed matrix of axes was transposed though the correct axes were used for the displayed ellipsoid. Also I was using inertia matrix I_ij = sum_over_k(m_k*r_ki*r_kj) omitting the diagonal term. Should have been I_ij = sum_over_k(m_k*(|r_k| - r_ki*r_kj)). And finally I incorrectly computed the ellipsoid size along each axis. Everything was a little bit wrong. Here's the fixed version. Tested in Chimera version 1.2500 but should work with earlier versions. Tom