[bug #9259] Reduced spectral density mapping yielding bad values -- June 01, 2007 - 17:15

URL:
  <http://gna.org/bugs/?9259>

                 Summary: Reduced spectral density mapping yielding bad
values
                 Project: relax
            Submitted by: None
            Submitted on: Friday 06/01/2007 at 17:15 CEST
                Category: relax's source code
                Severity: 4 - Important
                Priority: 5 - Normal
                  Status: None
                 Privacy: Public
             Assigned to: None
         Originator Name: Sébastien Morin
        Originator Email: sebastien.morin.1@xxxxxxxxx
             Open/Closed: Open
         Discussion Lock: Any
                 Release: Repository: 1.2 line
        Operating System: GNU/Linux

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Details:

Hi

I performed spectral density mapping on data recorded at three magnetic
fields (500, 600, 800).

The values I get are erroneous (when compared with Leo Spyracopoulos'
Mathematica notebook which were manually verified using equations from the
method 1 of Farrow et al., 1995, JBNMR, 6 : 153) and scaled depending on the
magnetic field as shown in the table below (for which values calculated using
either Leo's notebook or relax are divided by the value calculated manually).

Field    Method    J(0)          J(wN)         J(wH)
=====    =======   ==========    ==========    ==========
500      Farrow    1 (ref)       1 (ref)       1 (ref)
         Leo       1             1             1
         relax     0.04758       0.04757       0.999

600      Farrow    1 (ref)       1 (ref)       1 (ref)
         Leo       1             1             1
         relax     0.03361       0.03361       0.999

800      Farrow    1 (ref)       1 (ref)       1
         Leo       1             1             1
         relax     0.01932       0.01932       0.999

Then, if you take the different values for J(0) and J(wN) and compare from
field to field, you get this :

              J(0)      J(wN)     J(wH)
              ========  ========  ========
500/600 ->    1.415     1.415     1
500/800 ->    2.462     2.462     1

Those ratios are similar to what you get when comparing fields quadratically
:

(600/500)^2 = (1.2)^2 = 1.44 ~ 1.415
(800/500)^2 = (1.6)^2 = 2.56 ~ 2.462

So there seems to be a problem somewhere in the calculations of J(0) and
J(wN) and, to a lesser extent, J(wH)...

I first thought the problem was related with bug #9238... In fact, before
this bug was solved, the problem was worst by a factor of ~2... Still, the
skewing of Jw mapping results is quite important. Maybe is this something
with the units or constants values...

Thanks for helping me !


Sébastien :)






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