[gmueckl]

Writing tests for the the kind of numerical code that FORTRAN is usually used for is hard. Sometimes there is no direct way of testing it because if you knew any of the results already you wouldn't need to run the simulation in the first place. Quite often, the best that you can do is proper sanity checks like conservation of energy and momentum or things like that.

[fnord123]

Yes, it's hard. But without an automated test suite checking the numbers coming out then any change to the code could introduce numerical instability.