Before potential users start working with MOPAC, they must be reasonably sure that the program works as described. The testing of a new MOPAC is quite lengthy. The basic tests verify that all the functions work. This involves running many hundreds of systems, many of which are highly exotic. Once all tests of this type are successful, pairs of functions are tested. For example, two functions in MOPAC are (1) the ability to apply an external electric field, and (2) to optimize the geometry of a molecule. By combining these functions, the geometry of a molecular system can be optimized in the presence of an external electric field. Systems that would otherwise not interact can be made to weakly bond together under the influence of an external field. With the wide range of possible functions, testing all pairs of functions is not practical; however, most reasonable pairs of functions have been tested to ensure that they work correctly.

The number of possible combinations of three or more functions (an example of a three function calculation would be to calculate the path of an implanting ion as it approaches a solid surface while it is accelerating under the influence of a potential gradient). Several of these have been tested. There is no reason to assume that all such combinations of functions will work, but at the end of extensive testing, all calculations attempted did work.