> On the handling of atomic anisotropic displacement > parameters > R. W. Grosse-Kunstleve* and P. D. Adams > J. Appl. Cryst. (2002). 35, 477-480 > > to look up the above relationship. In the paper it is mentioned that > > |Ucart - (lambda)I|=0 > > is solved using Cardan's formula.
After the paper was published I discovered that my implementation based on Cardan's formula was numerically unstable in some cases. I found it too difficult to come up with a fully reliable implementation and eventually plugged in general eigenvalue code. I've been happy ever since. > So I probably oversimplified my solution and have it wrong. If your goal is to learn more about the eigenvalue problem, this page is great: http://en.wikipedia.org/wiki/Eigenvalue Look for "characteristic polynomial" (that's the equation you quote). If you just need to find the eigenvalues and vectors, the example script mentioned in the paper survived almost unchanged: http://cctbx.svn.sourceforge.net/viewvc/cctbx/trunk/cctbx/cctbx/examples/analyze_adp.py?view=markup You can install the cctbx in about 30 seconds by using one of the binary bundles here: http://cci.lbl.gov/cctbx_build/ Then: cctbx.python cctbx_sources/cctbx/cctbx/examples/analyze_adp.py Be sure to reorder the parameters you get from shelx! Shelx gives you U11 U22 U33 U23 U13 U12, while the cctbx uses the PDB order U11 U22 U33 U12 U13 U23. Ralf ____________________________________________________________________________________ Got a little couch potato? Check out fun summer activities for kids. http://search.yahoo.com/search?fr=oni_on_mail&p=summer+activities+for+kids&cs=bz
