… physicists ruin physics
(bumper sticker — I doubt if it exists, but it should)
The efficacy of a computer simulation (or of any other numerical computation) in predicting the behaviour of a physical system, crucially hinges on two ingredients:
1) The reliability of the underlying mathematical model adopted to describe the physical system of interest.
2) The accuracy of the numerical technique utilized.
This is just as true for physics as it is for any other field of inquiry — biology, economics, sociology, engineering, in fact for any research endeavour that relies on complex mathematical models, too intricate to be studied analytically.