double bigaus_function(double *x, double *p) { 
      double u = (x[0]-p[1])/p[2];
      double v = (x[1]-p[3])/p[4];
      double rho = p[5];
      double c = 1. - rho*rho;
      double z = u*u - 2.*rho*u*v + v*v;
      return  p[0]/(2 * TMath::Pi() * p[2] * p[4] * std::sqrt(c) )* std::exp(- z / (2. * c) );
}

void exampleBigaus() { 

   int n = 100000;
   int nbins = 50;
   double xmin = -10; 
   double xmax = 10; 
   TH2 * h2 = new TH2D("h1","h2",nbins,xmin,xmax, nbins, xmin, xmax);
   // for the ROOT master and  6.07.06
#if ROOT_VERSION_CODE >= ROOT_VERSION(6,7,6)   
   TF2 * f2 = new TF2("bigaus","bigaus");
#else
   // for older ROOT version
   TF2 * f2 = new TF2("bigaus",bigaus_function,xmin,xmax,xmin,xmax,6);
#endif

   f2->SetParameters(1,2,2,-1,3,0.3);

   h2->FillRandom("bigaus"); 

   //f2->SetParameters(100,2,2,-1,3,0.3);

   h2->Fit("bigaus"); 

   h2->Draw("COLZ"); 

}