geant4-DNA, chem6 EnergyLoss

Dear Geant4 experts, I want to simulate electron and proton irradiation by using Chem6, but i have an question about this example. in the Chem6 example for electron 1 MeV , why is energy Loss used to control and calculate G-Value?
I tried to define a 5 um cube of water Instead of defining energy loss , But in the following cases it gives a time-related error:
1- When the number of shooted particles in the target is increased. (it will work for 100 particles but will give an error for 1000 particles)
2- When the position of the source changes. (The position of the source changes from the edge to the center, it will give an error.)
would you please answer me what this error is?