I would like to fit isotropic exchange in trinuclear system, defined as
Sys.S = [3/2 3/2 3/2];
Sys.J = [J12 J13 J23];
and the question is whether it is possible to restrict J13=J23 relationship in ESFIT?
If yes, I would have only two varying parameters, J12 and J13 (J23=J13)...
Use your own custom Sys fields for the exchange couplings, e.g. Sys.J12 and Sys.J13, and then in the simulation function, set Sys.J = [Sys.J12 Sys.J13 Sys.J13] and call pepper. You can then vary Sys.J12 and Sys.J13 with esfit.