Difference between "G4_WATER" and user defined water with density=1g/cc and I_mean=78ev

Hi,
I am trying to study the effect of water composition on the Bragg peak for heavy ions. I have noticed that implementing the “G4_WATER” gives the accurate result compared to experimental data but I cannot get the same result with a user defined water like below:

G4Material* H2O = new G4Material(“Water”, 1.000g/cm3, nelements=2);
H2O->AddElement(elH, 2);
H2O->AddElement(elO, 1);
H2O->SetChemicalFormula(“H_2O”);
H2O->GetIonisation()->SetMeanExcitationEnergy(78.0
eV);

Additionally, I can see the effect of density variation but the effect of “I_mean” variation is negligible. I was wondering if this is the correct way to define the “I_mean” of water.
Thanks

Hi,
I would really appreciate any help I can get regarding this.
Thanks