In this way one can get 99% of the software functionality (indeed, I don't know the missing 1%, but I can imagine that it exists) and get the necessary experience with it together with useful results. The advantage of this approach is that you install, in principle, the executable once and than with each new official release do not risk possible interference of different new features in different packages.
So my suggestion is rather simple: start with atlsim and use it NOW. In parallel you can install other parts of the ATLAS software and debug it, but at least in case of a problem you can immediatly discover in which part it is - in a particular geometry/digitization discription or in the common atlas software itself.
You can get atlsim from Sasha's web page .
Please report your questions/comments/suggestions/successes/failures to me ( email@example.com ), Sasha ( firstname.lastname@example.org ), and Pavel ( Pavel.Nevski@cern.ch ).
Ransom W. Stephens