Principal Moment of Inertia--how to calculate? 1. The problem statement, all variables and given/known data I am writing a program that incorporates calculating the principal moment of inertia for a protein residue based on its component atom XYZ coordinates. I am exceedingly confused about which formulas to use in calculating principal moment of inertia for my situation. Thus far my program does the following: 1. Calculate elements (Ixx, Iyy, Izz, etc.) of symmetric matrix A 2. Find the eigenvalues (and thus the principal moments of inertia) 2. Relevant equations The formulas I originally used were from here: https://docs.google.com/viewer?a=v&q...ZFyxHNqw&pli=1 [Broken] However looking at Wikipedia, the formulas for the symmetric matrix elements are different. http://en.wikipedia.org/wiki/Moment_...tia#Definition [Broken] I have also been scouring the internet and found open source code where the center of mass is subtracted from the x, y, and z coordinates before beginning calculation of elements Ixx, Iyy, Izz, etc. 3. The attempt at a solution Which formulas/algorithm do I use to calculate principal moment of inertia for my case? I'm not sure where to begin with picking which formulas to use. Is there a source that is accessible to those with a weak physics background that will help me understand which formula/algorithm to use in calculating principal moment of inertia? Thank you!