'CPMG' or 'LM63' model parameter conversions.
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To convert the 'CPMG' model parameter Rex to phi_ex, use:

    phi_ex = (Rex / Tau) / (2.0 * pi * frq / g1H * g15N * 1e-6)**2 * 1000

where frq is the magnetic field strength in Hz (for the proton Larmor frequency).  The factor of 1000 is because of the ms units.

The kex parameter is given by:

    kex = 1.0/Tau * 1000

And tex as:

    tex = "%.3e" % (Tau / 1000)



'Full_CPMG' or 'CR72' model parameter conversions.
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To convert from the papb parameter:

    pA = 1.0 - papb

To obtain dw:

    dw = 2.0 * pi * dw

And kex:

    kex = kex * 1000



'Ishima' or 'IT99' model parameter conversions.
===============================================

To convert to the pA parameter:

    phi_ex = (Rex / Tau) / (2.0 * pi * frq / g1H * g15N * 1e-6)**2 * 1000
    pA = 1.0 - phi_ex / PaDw

To obtain dw:

    dw = sqrt(PaDw / pA)

The kex parameter is given by:

    kex = 1.0/Tau * 1000

And tex as:

    tex = "%.3e" % (Tau / 1000)
