Hi, The origin of this was exactly what Andy Baldwin said at http://thread.gmane.org/gmane.science.nmr.relax.devel/5410/focus=5448 - that there are many different tau_CPMG definitions and there is an error in one of the equations in the original paper. The code in relax is however correct. The eta_scale value should be 0.35355339059327373. Feel free to fix the equations in the documentation of the wiki, manual and lib.dispersion.cr72 docstring. I wrote these before the code was fully tested and hence there is now a mismatch. As for using 2.0**(-3.0/2.0) in the code, this is for the fastest operation, avoiding the square root. But now that I look at it, one mathematical operation can be dropped for speed and the factor changed to 2.0**(-1.5).
Cheers, Edward On 3 May 2014 12:28, Troels Emtekær Linnet <[email protected]> wrote: > Dear Edward. > > I was inspecting the code of model CR72. > > http://svn.gna.org/viewcvs/*checkout*/relax/trunk/lib/dispersion/cr72.py?revision=HEAD > > This is calculated as: > > eta_scale = 2.0**(-3.0/2.0) = 0.35 > which is: 1 / sqrt(2**3). > > This differs from the definition in: > http://wiki.nmr-relax.com/CR72_full#Equation > http://www.nmr-relax.com/manual/full_CR72_2_site_CPMG_model.html > http://www.nmr-relax.com/api/3.1/lib.dispersion.cr72-module.html > > where: > eta_scale = 2.0**(2/3) = 1.58 > which is {3}sqrt(2**2). > > If I compare to: > http://wiki.nmr-relax.com/B14#Equation_compared_to_Carver_Richards_72 > (DOI: 10.1016/j.jmr.2014.02.023 , eq 70,) > > And to, (DOI: 10.1016/S0076-6879(01)39315-1 , eq 25, with the twist > that "tcp_Pa is the delay between 180 pulses in the CPMG pulse train", > "delay/180/delay/delay/180/delay " so that tcp_Pa = 2xdelay > > Then really the manual should use: > eta_scale = 2.0**(-3/2) > or > 1 / 2 sqrt(2) > > _______________________________________________ > relax (http://www.nmr-relax.com) > > This is the relax-devel mailing list > [email protected] > > To unsubscribe from this list, get a password > reminder, or change your subscription options, > visit the list information page at > https://mail.gna.org/listinfo/relax-devel _______________________________________________ relax (http://www.nmr-relax.com) This is the relax-devel mailing list [email protected] To unsubscribe from this list, get a password reminder, or change your subscription options, visit the list information page at https://mail.gna.org/listinfo/relax-devel
