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Hi, as beginner you may want to have a look at the documentation first... http://cern.ch/geant4/UserDocumentation/UsersGuides/ForApplicationDeveloper/html/ch03s02.html#sect.GlobClass.HEPRandom > CLHEP::HepRandom::setTheEngine(new CLHEP::RanecuEngine); > > is used to instantiate a static random engine, which will internally set a seed. Right. This shows you how to set a random engine from the many engines available in CLHEP. Each engine has its default initial seed. > This same seed will be used in every run, which is why the results are all same > in every run. But a different seed will be provided if the program is recompiled. No. Recompilation of the program will not change the random sequence. The random sequence would change only if you modify the initial seed of the engine. Although it is not a recommended practice, some applications may do this by initialising the seed with the system time, so one gets different results each time the application is executed... The seed can either be changed directly in the code (and in this case, as RanecuEngine is used, it can be represented by a couple of numbers), using: HepRandom::SetTheSeeds(seeds); // 'seeds' is an array of two positive numbers X,Y you choose or at run-time from the application's prompt (or adding it to the input macro, here run.mac), using the command: /random/setSeeds X Y You should be able to find all this in the documentation. Gabriele