Hi, Just so you know, the first numerical model has been implemented in relax. This is the one using the complex conjugate matrices from http://thread.gmane.org/gmane.science.nmr.relax.devel/4132. You can see the code in the files: lib/dispersion/ns_2site_star.py lib/dispersion/ns_matrices.py lib/linear_algebra/matrix_power.py target_functions/relax_disp.py specific_analyses/relax_disp/api.py specific_analyses/relax_disp/variables.py The first 3 contain the code from the 'fitting_main_kex.py' file attached to https://gna.org/task/?7712#comment3 (https://gna.org/support/download.php?file_id=18263), and the above link. These will require checking and authorship corrected or added to the copyrights. The last 3 contain code I wrote to support the model and its optimisation (most of it was to support the R2A and R2B parameters as this was not handled by relax before). You can see all the changes by typing in the prompt and base directory of the relax_disp branch the following: $ svn up; svn log -v | less Look at revisions r20269 to r20289. The only thing not implemented yet are system/unit/GUI tests. However if you run: ./relax -s Relax_disp.test_hansen_cpmg_data_auto_analysis you will see this model applied to a truncated set of CMPG data from Flemming Hansen. You will also see that there is still some debugging (parameter description and mapping issues) to do before this is fully functional. Regards, Edward