Hi all,

this is tricky but I need to understand what to do… I have a root macro which perform a fit of a function with the following code:

```
RooRealVar p1_0_Sig("p1_0_Sig","p1_0_Sig",0.0001, 1.0);
RooRealVar p1_1_Sig("p1_1_Sig","p1_1_Sig",0.1 ,100.);
RooRealVar p1_2_Sig("p1_2_Sig","p1_2_Sig",0.01 ,100.);
RooGenericPdf f_bkgd_1_Sig("f_bkgd_1_Sig", "f_bkgd_1_Sig","p1_0_Sig * pow((1-mInv/13000),p1_1_Sig)/pow(mInv/13000,p1_2_Sig)",RooArgList(mInv,p1_0_Sig,p1_1_Sig,p1_2_Sig));
auto result_1_Sig = f_bkgd_1_Sig.fitTo(hist_bkgd1_Sig,SumW2Error(true),PrintLevel(printlevel),RooFit::Save(true));
result_1_Sig->Print();
p1_0_Sig.setConstant(kTRUE);
p1_1_Sig.setConstant(kTRUE);
p1_2_Sig.setConstant(kTRUE);
```

Sometimes I work on a linux system:

```
Linux ui02.recas.ba.infn.it 3.10.0-1160.80.1.el7.x86_64 #1 SMP Tue Nov 8 15:48:59 UTC 2022 x86_64 x86_64 x86_64 GNU/Linux
```

with the CERN environment: /cvmfs/sft.cern.ch/lcg/views/LCG_105/x86_64-centos7-gcc11-opt and ROOT 6.30/02

Sometimes I work on my Mac (Sonoma 14.5) with ROOT 6.30/06.

The result of the fit are different (this is just one example… I use this function and other similar function hundreds of times and the overall behavior is really different)

Linux output:

```
[#1] INFO:NumericIntegration -- RooRealIntegral::init(f_bkgd_1_Sig_Int[mInv]) using numeric integrator RooRombergIntegrator to calculate Int(mInv)
[#1] INFO:Minimization -- RooAbsMinimizerFcn::setOptimizeConst: activating const optimization
[#1] INFO:Fitting -- RooAbsPdf::fitTo(f_bkgd_1_Sig) Calculating sum-of-weights-squared correction matrix for covariance matrix
[#1] INFO:Minimization -- RooAbsMinimizerFcn::setOptimizeConst: deactivating const optimization
RooFitResult: minimized FCN value: 69259.6, estimated distance to minimum: 12.35
covariance matrix quality: Approximation only, not accurate
Status : MINIMIZE=0 HESSE=0 HESSE=0
Floating Parameter FinalValue +/- Error
-------------------- --------------------------
p1_0_Sig 5.0005e-01 +/- 8.40e-07
p1_1_Sig 1.4312e+01 +/- 8.82e-01
p1_2_Sig 4.0143e+00 +/- 2.08e-02
```

OSX output:

```
[#1] INFO:NumericIntegration -- RooRealIntegral::init(f_bkgd_1_Sig_Int[mInv]) using numeric integrator RooRombergIntegrator to calculate Int(mInv)
[#1] INFO:Minimization -- RooAbsMinimizerFcn::setOptimizeConst: activating const optimization
[#1] INFO:Fitting -- RooAbsPdf::fitTo(f_bkgd_1_Sig) Calculating sum-of-weights-squared correction matrix for covariance matrix
Warning in <ROOT::Math::Fitter::CalculateHessErrors>: Error when calculating Hessian
[#1] INFO:Minimization -- RooAbsMinimizerFcn::setOptimizeConst: deactivating const optimization
RooFitResult: minimized FCN value: 99020, estimated distance to minimum: 0.000405247
covariance matrix quality: Not calculated at all
Status : MINIMIZE=0 HESSE=0 HESSE=300
Floating Parameter FinalValue +/- Error
-------------------- --------------------------
p1_0_Sig 5.0005e-01 +/- 4.63e+04
p1_1_Sig 1.4378e+01 +/- 3.43e+00
p1_2_Sig 4.0132e+00 +/- 8.05e-02
```

How do you explain this difference? Just to give you an idea of my problem… on my OSX the fit gives a STATUS=0 only ~60 times on more than 2500 different samples histogram on the Linux system STATUS=0 is more than 1800 over the same 2500 histogram…

Another example…

OSX:

```
RooFitResult: minimized FCN value: 98901.3, estimated distance to minimum: 3.64921
covariance matrix quality: Full, accurate covariance matrix
Status : MINIMIZE=0 HESSE=0 HESSE=0
Floating Parameter FinalValue +/- Error
-------------------- --------------------------
p1_0_Muo 5.0005e-01 +/- 4.63e+04
p1_1_Muo 1.7899e+01 +/- 3.38e+00
p1_2_Muo 3.9459e+00 +/- 7.77e-02
```

LINUX:

```
[#1] INFO:NumericIntegration -- RooRealIntegral::init(f_bkgd_1_Muo_Int[mInv]) using numeric integrator RooRombergIntegrator to calculate Int(mInv)
[#1] INFO:Minimization -- RooAbsMinimizerFcn::setOptimizeConst: activating const optimization
[#1] INFO:Fitting -- RooAbsPdf::fitTo(f_bkgd_1_Muo) Calculating sum-of-weights-squared correction matrix for covariance matrix
[#1] INFO:Minimization -- RooAbsMinimizerFcn::setOptimizeConst: deactivating const optimization
RooFitResult: minimized FCN value: 69228.4, estimated distance to minimum: 3.10305e-06
covariance matrix quality: Approximation only, not accurate
Status : MINIMIZE=0 HESSE=0 HESSE=0
Floating Parameter FinalValue +/- Error
-------------------- --------------------------
p1_0_Muo 2.5491e-02 +/- 1.28e+00
p1_1_Muo 1.7768e+01 +/- 1.08e+00
p1_2_Muo 3.9490e+00 +/- 2.51e-02
```

In this case both have STATUS=0 but numbers are different and also errors are different…

Any idea of what’s going on?

Thanks

Attilio