Experiments provide, through observation of physical reality, insight into processes of unknown mechanism, allowing deductions regarding the physical nature of the observed phenomenon. Typically, everything is known about the microscopic processes, but nothing is known a priori about the way in which macroscopic observables depend on these processes. The result can be as unexpected as in a true experiment. The advantage of simulations lies inthe possibility to inquire about the effect of microscopic features on macroscopic observables in a systematic way, especially where no theoretical connection is known. Thus one should be careful to distinguish between reality, models of reality, and the behavior of these models in relation to real behavior.
Experiments yield measurements of real behavior.
Theory invokes a model of reality and a number of approximations to render the behavior of the model mathematically tractable. The computer simulations used by engineers to estimate material properties involve many atoms and molecules and are usually performed assuming the Laws of Classical Mechanics . The sytems simulated are conservative, meaning that state functions like energy are conserved. Usually, one obtains a time evolution at constant energy, consistent with the integration of equations of a conservative system. Usually, this sampling is weighted according to the Boltzmann factor in order to produce canonical averages.