Problems with the contour using TMinuit

Hello ROOTers,

I'm trying to get a contour using TMinuit but I'm having some problems. If I define limits for my 2 parameters very close to their minimum, I get good errors and the contour is drawn. But this contour is squeezed by the parameter limits, regardless of the number of points:

  MINUIT WARNING IN MNContour

• Contour squeezed by parameter limits.

If I try to put the limits more far away from the minimum, having a larger range for the parameters, the errors for them are very large and I get no contour:

 MNCONT UNABLE TO FIND FOUR POINTS.
Warning in <TMinuit::Contour>: Cannot find more than 4 points, no TGraph returned

Does anyone know how can I manage to draw the right contour in this case? I must say that my FCN has several local minimums.

Thanks in advance!

Here follows part of my code:

void myFit_test_3(){

  (...) some definitions (...)

  TMinuit* minuit = new TMinuit(npar);

  Double_t arglist[10];
  Int_t ierflg = 0.;
  arglist[0] = 0.5;
  minuit->mnexcm("SET ERR", arglist ,1,ierflg);

  minuit->SetFCN(fcn);

  double par1 = 0.945414;
  double par0 = 2.6235E-3;
  Double_t step[2] = {0.0001,0.001};
  Double_t vstart[2] = {par0,par1};

  Double_t slimits[2] = {0.9,1.0};
  Double_t Dlimits[2] = {0.0015,0.003};

  minuit->mnparm(0, "Dm", vstart[0], step[0], Dlimits[0],Dlimits[1],ierflg);   minuit->mnparm(1, "sin", vstart[1], step[1],slimits[0],slimits[1],ierflg);

  arglist[0] = 500;
  arglist[1] = 1.;
  minuit->mnexcm("MIGRAD", arglist ,2,ierflg);

  minuit->GetParameter(0,Dmfit,Dmfit_err);   minuit->GetParameter(1,sfit, sfit_err);

  TH2D* conthist = new
TH2D("conthist","Contours",100,smin,smax,100,Dmmin,Dmmax);   TCanvas* ccan = new TCanvas("ccan","Contour Canvas",650,650);   conthist->Draw();

  gMinuit->SetErrorDef(1);
  TGraph* interpcont1sig = (TGraph*)minuit->Contour(10,1,0);   Int_t ge = minuit->GetStatus();
  cout<<"Contour Result = "<<ge<<endl;
  interpcont1sig->Draw("ALP");
}

Cheers,
Michelle Mesquita de Medeiros. Received on Mon Feb 20 2012 - 22:39:05 CET