Geant4 and SRIM

So I've spent a while trying to get Geant4 to do ion implantations similar to SRIM. Turns out Example TestEm7 models a particle (can be an ion -- see the included C12 macro) impacting a block of material, and even includes some extra physics to make the "low-energy" ion calculation more accurate (screened nuclear stopping, custom-defined physics list standardNR, by Mendenhall and Weller). It calculates the predicted range of such ions.

Using Geant4.9.3.b01 and the standardNR physics list, my quick accuracy test gave an ion range for P-31 at 10keV (very low energy in Geant4 terms) into NIST-parameterized Si of 15nm versus SRIM-2008's 17nm (monolayer mode). Both give rms values of about 5-6nm.

To get energies and positions of transmitted particles requires some editing of the C++ code.

Why am I doing this? SRIM only does layers of material, I'm trying to model a more complicated geometry.