FCN=3.70998e+09
Don’t you think that your chi^2 is much too high?
Maybe your fit-function is not able to reproduce your data well?
You could also try to change the default minimizer / fitter … // ROOT::Math::MinimizerOptions::SetDefaultMinimizer("Minuit");
// available Minuit2 algorithms: "Migrad", "Simplex", "Combined", "Scan", "Fumili"
// ROOT::Math::MinimizerOptions::SetDefaultMinimizer("Minuit2", "Combined");
// ... or ...
// TVirtualFitter::SetDefaultFitter("Minuit");
// TVirtualFitter::SetDefaultFitter("Minuit2"); // the default algorithm is "Migrad" … and/or tolerance and/or precision … see web links in this post: [url]Tweaking the ROOT fitting function
BTW. You “delete func;” but this “func” is still used by your “geff” (in the list of functions of this graph -> expect problems if you “plot” it, for example).