http://www.cryst.bbk.ac.uk/PPS2/course/section9/index.html
http://www.dl.ac.uk/PPS/course/section9/index.html
http://www.pdb.bnl.gov/PPS2/course/section9/index.html
http://www.man.poznan.pl/PPS/course/section9/index.html
http://www.biochemtech.uni-halle.de/PPS2/course/section9/index.html
See also
http://www.cryst.bbk.ac.uk/PPS2/assignments/week18.html