Dear RooFit-experts,

I have just started to use RooFit, but am somehow stuck.

What I want to do is fitting distributions with an “exponentially modified Gaussian distribution” (http://en.wikipedia.org/wiki/Exponentially_modified_Gaussian_distribution). For this I have implemented my own pdf class (GaussExpConv), using the RooClassFactory. I implemented the evaluate() method accordingly, and I can plot the resulting pdfs with predefined parameters - they look reasonable.

```
Double_t GaussExpConv::evaluate() const
{
double val1 = exp(0.5*tau*(2*mean+tau*sig*sig-2*x));
double val2 = erfc((mean+tau*sig*sig-x)/(sqrt(2)*sig));
return 0.5*tau*val1*val2;
}
```

However, fitting fails: In my python macro I do

```
rooFits[k] = ROOT.GaussExpConv("rooFit_%d"%k,"rooFit_%d"%k,rooX,rooMean,rooSig,rooTau)
rooMean.Print()
result = rooFits[k].fitTo(rooHist,
ROOT.RooCmdArg("Optimize",False),
ROOT.RooCmdArg("Save",True))
rooMean.Print()
result.Print()
```

The fits converge after ~50-100 calls, there are no errors printed, but the parameters are unchanged. The fit result is empty.

```
FCN=111.756 FROM MINOS STATUS=SUCCESSFUL 21 CALLS 155 TOTAL
EDM=8.79738e-08 STRATEGY= 1 ERROR MATRIX ACCURATE
EXT PARAMETER STEP FIRST
NO. NAME VALUE ERROR SIZE DERIVATIVE
1 p0 1.67404e+02 4.90268e+00 -3.20256e-02 3.33877e-05
2 p1 1.07053e+00 4.04102e-03 1.52869e-05 6.91878e-02
3 p2 1.50264e-01 2.75660e-03 2.75660e-03 -2.60437e-01
RooRealVar::rooMean_22 = 1.07967 C L(-INF - +INF)
[#1] INFO:NumericIntegration -- RooRealIntegral::init(rooFit_22_Int[p]) using numeric integrator RooIntegrator1D to calculate Int(p)
RooFitResult: minimized FCN value: -554.283, estimated distance to minimum: -999
covariance matrix quality:
Status :
Floating Parameter FinalValue +/- Error
-------------------- --------------------------
```

First suspecting the erfc implementation, I changed the evaluate() method to return a simple TMath::Gaus, with the same effect: Fitting does not alter the model parameters.

When replacing the custom PDF with one of the predefined, e.g. the RooGaussian, however, the fit has the expected effect. What is going wrong? Why is there no error printed? How can I get more feedback?

Looking forward for your input,

Q.