*ROOT Version:* 6.28/06

*Platform:* macosxarm64

*Compiler:* clang-1403.0.22.14.1

Hi,

As the title suggested, I’m seeing some strange differences in the fit results when using custom-defined functions as opposed to ROOT’s predefined functions, despite the functions being mathematically equivalent.

Here’s my macro:

```
Double_t Gaussian(Double_t* x, Double_t* par) {
// Parameters:
// par[0]: amplitude
// par[1]: mean
// par[2]: standard deviation (width)
return par[0]*exp(-0.5*((x[0]-par[1])/par[2])*((x[0]-par[1])/par[2]));
}
Double_t LinearBackground(Double_t* x, Double_t* par) {
// Parameters:
// par[0]: y-intercept
// par[1]: slope
return par[0] + par[1]*x[0];
}
Double_t TotalFunction(Double_t *x, Double_t *par) {
return Gaussian(x,&par[0]) +
Gaussian(x,&par[3]) +
LinearBackground(x,&par[6]);
}
void find_my_data(Double_t xmin, Double_t xmax)
{
// Reset all existing global objects in ROOT to start with a clean slate
gROOT->Reset();
// Read in a 1D histogram
TFile *inputFile = TFile::Open("/path/to/file/file.root","READ");
TH1F *xavg_hist = (TH1F*)inputFile->Get("xavg_1d");
for (int i = 1; i <= xavg_hist->GetNbinsX(); i++) {
double entries = xavg_hist->GetBinContent(i);
xavg_hist->SetBinError(i, sqrt(entries));
}
// Defining parameters to use in both fits
Double_t pars[8] = {51.1, -6.8, 0.6, 1158., -10.76, 1.17, 85.6, -2.1};
// Fit with custom function
TF1 *f = new TF1("f", TotalFunction, xmin, xmax, 8);
f->SetParameters(pars);
xavg_hist->Fit(f,"R");
// Second Fit: Using ROOT's Predefined Functions
TF1 *f_predefined = new TF1("f_predefined", "gaus(0)+gaus(3)+pol1(6)", xmin, xmax);
f_predefined->SetParameters(pars);
xavg_hist->Fit(f_predefined, "R");
gStyle->SetOptFit(1111);
// Change the appearance of the second fit function
f->SetLineColor(kBlack);
f->SetLineStyle(2);
f->SetLineWidth(2);
xavg_hist->Draw();
f->Draw("same");
f_predefined->Draw("same");
}
```

Attached is an image of the fit results, with the fit using ROOT’s predefined functions shown in red. As you can see here, the predefined function fit appears to be more reasonable with the data compared to the custom function fit.

I have ensured (as best as I can) that the same parameters and settings are used for both fits. At this point, I’m not quite sure if there is something else I can check. Please let me know if you notice a mistake in my macro, or if this is a bug.