The missing Diso parameter bug #5559, reported at https://gna.org/bugs/index.php?func=detailitem&item_id=5559 has been fixed. Now the relax test suite element 'Test of the user function diffusion_tensor.display()' passes. Chris, I've borrowed from your ideas introduced in trying to solve bug #5501 at https://gna.org/bugs/index.php?func=detailitem&item_id=5501. The non-standard diffusion tensor parameters are generated on the fly using the function __getattr__ and logic similar to what is in your code. If you look at the new code in 'data.py', the commit message with credit for your ideas is at https://mail.gna.org/public/relax-commits/2006-03/msg00058.html, you'll see that it is very similar to your code. Edward