Double_t g2(Double_t *x, Double_t *par)
  /* 2-dim Gaussian 
   par0 = normalisation
   par1=mu_1
   par2=sigma_1
   par3=mu_2
   par4=sigma_2
   par5=rho
   */
{
if(par[2] > 0 && par[4] > 0)
{
 double norm=par[0]/(8*atan(1.)*par[2]*par[4]*sqrt(1.-par[5]*par[5]));
 double rx=(x[0]-par[1])/par[2];
 double ry=(x[1]-par[3])/par[4];
 double cov=2*par[5]*rx*ry;
 return norm*TMath::Exp(-(rx*rx+ry*ry-cov)/(2.*(1-par[5]*par[5])) );
}
 else
   return 0;
}

void gaus2d() 
{
 TF2 *g2 =new TF2("gaus2", g2, -3.,3.,-3.,3.,6);
 g2->SetParameter(0,1.); // normalisation
 g2->SetParameter(1,0.); // mu1
 g2->SetParameter(2,1.); // sigma1
 g2->SetParameter(3,0.); // mu2
 g2->SetParameter(4,1.); // sigma2 
 g2->SetParameter(5,0.7); // rho
 // draw 2-dim function
  g2->Draw("surf3");
}