{ 
   TH1D * h1 = new TH1D("h1","h1",100,-5,5); 
   h1->FillRandom("gaus"); 
   TVirtualFitter::SetDefaultFitter("Fumili2"); 

   TF1 * f1 = new TF1("f1","[0]*TMath::Gaus(x,[1],[2])"); 
   f1->SetParNames("Amplitude","mean","sigma"); 
   f1->SetParameters(100,0,1); 
   f1->FixParameter(0,100);

   h1->Fit(f1); 
   
}