Thank you for your detailed post.
In the meantime, I think SRIM implements the Continuous Slowing Down Approximation (CSDA), so for a more meaningful comparison it is usually recommended to prevent the production of delta rays and disable energy loss fluctuations in Geant4 [1]. This helps mimic SRIM’s deterministic treatment of energy loss and avoids differences arising from stochastic effects or secondary particle production.
To do so, from the EM example test18 [2], you can use the following commands:
# prevent any delta rays production
/run/setCut 100 km
# disable fluctuations
/process/eLoss/fluct false
I am not an expert on the topic, so I let the experts give a proper answer, but I hope these indications go into the right direction.
Best,
Alvaro
PS: I found other effects that may play a role, for example the step limitation by energy loss [3]; in case you want to try more stringent parameters, you can set it as follows (you may try other values)
/process/eLoss/StepFunction 0.1 0.001 mm
As it was discussed in the previous Geant4 thread, MSC may have an impact as well.
[1] Geant4 and SRIM radon-222 range agreement - #2 by guatelli
[2] geant4/examples/extended/electromagnetic/TestEm18/csda.mac at master · Geant4/geant4 · GitHub
[3] Mean Energy Loss — Physics Reference Manual 11.3 documentation