[bug #22210] The 'LM63 3-site' relaxation dispersion model does not work. -- June 17, 2014 - 14:01

URL:
  <http://gna.org/bugs/?22210>

                 Summary: The 'LM63 3-site' relaxation dispersion model does
not work.
                 Project: relax
            Submitted by: bugman
            Submitted on: Tue 17 Jun 2014 12:01:08 PM UTC
                Category: relax's source code
Specific analysis category: Relaxation dispersion
                Priority: 5 - Normal
                Severity: 4 - Important
                  Status: None
             Assigned to: bugman
         Originator Name: 
        Originator Email: 
             Open/Closed: Open
                 Release: 3.2.2
         Discussion Lock: Any
        Operating System: All systems

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Details:

The Luz and Meiboom 1963 3-site 'LM63 3-site' relaxation dispersion CPMG model
(http://wiki.nmr-relax.com/LM63_3-site) is buggy.  This can be seen in the
source code directly:

http://www.nmr-relax.com/api/3.1/lib.dispersion.lm63_3site-pysrc.html

The formula is:

                  _3_
                  \    phi_ex_i   /     4 * nu_cpmg         /     ki      \ \

   R2eff = R20 +   >   -------- * | 1 - -----------  * tanh | ----------- | |
.
                  /__     ki      \         ki              \ 4 * nu_cpmg / /
                  i=2


But the code implementing the tanh(ki/(4*nu_cpmg)) is:

            back_calc[i] += rex_B * (1.0 - cpmg_frqs[i] * tanh(quart_kB *
cpmg_frqs[i]) / quart_kB)


Clearly the multiplication in the tanh function must be a division!




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