In this section, the user can select a model for fitting the data based on a chosen exchange regime.
The fittings presume the following kinetic scheme:
The following formulas are used to calculate R_{2,eff} for different models such as the BlochMcConnell [1], CarverRichards [2, 3] and LuzMeiboom [4] models:
BlochMcConnell model 

CarverRichards model 

LuzMeiboom model 

where 
T_{CP}  given constant relaxation delay 
 given CPMG frequency 
B_{0}  given field strength for the nuclei of the interest in MHz 
 
R_{2}  fitted intrinsic transverse relaxation rate 
k_{AB}, k_{BA}  fitted kinetic rate constants (slow exchange) 
k_{ex}  fitted kinetic rate constant (fast exchange) 
Δδ  fitted chemical shift difference in ppm (slow exchange) 
φ  fitted population weighted chemical shift difference in ppm^{2} (fast exchange) 

The optimization of the parameters is performed by minimizing the target function:
where 
 given experimental relaxation rate 
 given experimental error in the relaxation rate 
 calculated relaxation rate based on model 

References
[1]  McConnell, H. M. (1958) Reaction rates by nuclear magnetic resonance, J. Chem. Phys. 28, 430 431 
[2]  Davis, D.G. et al. (1994) Direct measurements of the dissociationrate constant for inhibitorenzyme complexes via the T1 rho and T2 (CPMG) methods. J. Magn. Reson. B, 104, 266–275. 
[3]  Carver, J. P.; Richards, R. E. (1972) General 2site solution for chemical exchange produced dependence of T2 upon CarrPurcell pulse separation J. Magn. Reson., 6, 8996. 
[4]  Luz, Z. and Meiboom, S. (1963) Nuclear Magnetic Resonance study of the protolysis of trimethylammonium ion in aqueous solution—order of the reaction with respect to solvent. J. Chem. Phys., 39, 366–370. 